Effect Of Multi-electromagnetic Fields On The Quality Of Continuous Casting Hollow Billet

In recent years, with the development of industry and national economy, the needs of high quality large-diameter steel and copper alloy hollow tube and the requirement for the high production efficiency increase quickly. Continuous casting of hollow billet is considered as a profound near-net forming technology with the advantage of high production efficiency and high utilization ratio. But there are many problems for the hollow billet such as poor surface quality, too many defects, and coarse grains in solidification structure and poor uniformity of structure in the circumferential direction. How to produce high quality large-diameter hollow billet to satisfy the requirements of succeeding treatment is the key to acquire the high quality large-diameter hollow tube. The successful application of electromagnetic field in material processing provides a chance for producing high quality hollow billet.The purpose of the dissertation is to produce high quality large-diameter hollow billet. The study of the effect of multi-electromagnetic field on the quality of hollow billet is proceeding and the main study contents are as follows:The electromagnetic field and flow field of the molten metal with the electromagnetic stirrer were simulated by ANSYS software. The influence of geometric parameters, including winding type, number of pole-pairs, yoke size, and the current parameters, including current intensity and current frequency on the molten melt Sn-3.5%Pb were investigated. As a result, the optimization rules for the stirrer design were obtained in order to increase the electric efficiency, and the two kinds of electromagnetic stirrers-traveling magnetic field and rotating magnetic field were designed based on the rules.The low melting Sn-3.5%Pb alloy is used insteading of high melting steel to do the static simulation experiment of large-diameter hollow billet vertical continuous casting by imposed the electromagnetic stirrer in the inner-mold. The experimental results show that electromagnetic stirrer can refine grains and improve the uniformity of structure in the circumferential direction, the bigger the current intensity is, the better the solidification structure of hollow billet.In order to analysis the refining mechanism of hollow billet solidification structure, the software ANSYS and Fluent are used to compute the couple model of two-phase flow solidification and electromagnetic field to acquire the influence of stirring magnetic field on the solidification process of Sn-3.5%Pb, and the effect of the rotating magnetic field and traveling magnetic field on the temperature field, concentration field and grain size of hollow billet are analyzed. The following results can be acquired: Both stirring magnetic fields can decrease the rate of cooling and temperature gradient during crystallization of Sn-3.5%Pb, which is one of the reasons for the grain refining. The bigger the stirring velocity is, the smaller the grain diameter; however, the electromagnetic stirrer with bigger stirring velocity can result in the macro-segregation. So, the stirring intensity should be controlled when the electromagnetic stirrer is imposed.In addition, the electromagnetic volume forces of molten metal with the traveling magnetic field and rotating magnetic field were computed, the results show that both the electromagnetic volume forces are pulsating force with the frequency ofω/π. The pulsating force can strengthen the turbulence of molten metal and speed-up the heat dissipated which is benefic to the form of equiaxed grain solidification structure.In order to further improve the surface quality of hollow billet, the multi-electromagnetic fields are imposed during the vertical continuous casting of large-diameter hollow billet. The stirring magnetic field is imposed in the inner-mold and the middle frequency magnetic field is imposed in the outer-mold. The following merits can be acquired by the multi-electromagnetic fields: 1) The stirring magnetic field can not only improve the solidification structure of the large-diameter hollow billet but also eliminate the inclusion and blow hole flaws and improve the inner-surface quality of the hollow billet to avoid the milling of the inner-surface. 2) The middle frequency magnetic field can restrain the segregation burl and ripple mark of inner and outer surface of hollow billet. 3) The temperature field numerical simulation of electromagnetic continuous casting of the hollow billet shows that: Both the only medium frequency coil and the only stirring magnetic field are easy to create the crack flaw because of the non-homogeneous of the solidification shell. The imposed multi-electromagnetic fields can not only improve the homogeneous of solidification shell to restrain the crack flaw but also speed-up the heat dissipation to decrease the melt pool and improve the production efficiency.