Studies Of Soft-Contact Electromagnetic Continuous Casting Process Under Intermediate Frequency Electromagnetic Field

The technology of soft-contact electromagnetic continuous casting of steel is used to improve the surface quality of billets by imposed an alternating electromagnetic field to control the initial solidification of liquid steel. In recent years, the technology research has been focus on the soft contact electromagnetic continuous casting under high-frequency electromagnetic field. However, the manufacture of high-frequency power and mold are very difficult and high cost, high-frequency alternating magnetic field has a strong skin effect which causes poor penetrating and huge energy losing, disturb the normal working of the electronic devices of the caster, the industrial promotion of soft-contact electromagnetic continuous casting very difficult, and so inadequate. In this paper, the technology of using intermediate frequency electromagnetic fields in place of high-frequency electromagnetic field has been put forward and carry out the investigation of the electromagnetic field, flow field, heat transfer, solidification, meniscus behavior and the influence of different electrical parameters, structure parameters of mold and process parameters on them in order to provide theoretical and technical fundamention for industrial applications.Based on the theoretical analysis, the three-dimensional electromagnetic field finite element model of intermediate frequency electromagnetic soft-contact continuous casting mold is developed and the three-dimensional distribution of magnetic induction intensity, electromagnetic volume force, induced current and Joule heat in mold are revealed. The results show that:under the intermediate frequency magnetic field, the magnetic induction intensity component along the casting direction in the mold is much larger than ones in the direction of radial component and circumferential component, so that the magnetic pressure component along the radial is much larger than that in the other two directions, which shows that the characteristic of magnetic filed in the mold with imposed intermediate frequency magnetic fields has the same characteristics of electromagnetic pressure as high-frequency magnetic field. The magnetic field mainly concentrated in the high range of the coil along the casting direction, the position of maximum magnetic induction intensity is near the center height of the coil, and the magnetic induction intensity rapid decrease along the both ends of mold. With the frequency of 2500Hz and current intensity of 800A, the maximum magnetic induction intensity is 0.07261T, which meets the requirements of soft-contact technology. The distributions of magnetic induction intensity are same in slit and segment center, but the maximum magnetic induction intensity in segment center is slightly less than that in slit. The magnetic induction intensity linear increases with the increasing of current intensity in coils, it also increases with the increasing of frequency, but the increasing amplitude decrease. When the frequency is above 2500Hz, the uniformity of magnetic induction intensity along the circumferential direction will gradually deteriorate. The slits width has little effect on the distribution of magnetic induction intensity along the casting direction in mold, so the slits width of 0.5mm-0.8mm is recommended. With the increasing of slits number, the shielding of mod to intermediate frequency magnetic field is reduced, the magnetic induction intensity in outer surface of molten steel increases, but the growth rate gradually decrease. When the slits number is 8, the uniformity of magnetic fields along the circumferential direction of mold under intermediate frequency (2500Hz) is as the same as that in mould with slits number of 12 under high-frequency (20.5 kHz). With the height of free surface declining from the top of coils, the magnetic induction intensity in outer surface of molten steel firstly increases and then decreases. The maximum magnetic induction intensity can be gotten when the position of free surface is at the center height of coils. With the height of free surface declining, the non-uniformity of magnetic field distribution increases. With the relative position of the coils declining, the distribution of magnetic field along the casting direction doesn’t change, but the effective region of magnetic field declines, the maximum magnetic induction intensity decreases slightly.From the Bennon’s continuum model, a three dimensional coupled mathematical model of flow field, heat transfer and solidification in mold under intermediate frequency electromagnetic soft-contact continuous casting is developed. The fluid flow and temperature distribution of molten steel are revealed under different parameters. The results show that: intermediate frequency electromagnetic field mainly has effects on the meniscus zone of billet, which induce the incensement of fluid velocity and temperature of molten steel, the position declining of initial solidification point and thin shell thickness of initial solidification, and it cannot act on molten steel in the lower part of mold. The flow velocities are periodical antisymmetric distribution at the center of slit and segment which results in the concavo-convex deformation at free surface. There are two vortexes forming in longitudinal section compared to the one vortex in conventional continuous casting. The flow velocity and temperature increase with the current intensity increasing in the upper part of mold. When frequency increases, the magnitude and direction of fluid velocity vector also altered, meniscus gradually stabilizes, the temperature of molten steel in three-phase-point near the free surface increases slightly, the fluctuation of free surface of liquid steel could be retrained under the higher frequency electromagnetic field.Based on the numerical simulation, compared the calculation results, experimental study of electromagnetic field distribution and meniscus behavior was carried out forφ100mm round billet under intermediate frequency soft contact electromagnetic continuous casting, revealed the rule of electromagnetic field distribution, meniscus deformation and meniscus fluctuations under different parameters. The experimental results show that:the meniscus height was very closed along the circumferential direction, meniscus height in segment slightly lower than that in the slit, the meniscus height increased with the power increasing and the fluctuations of meniscus become more intense, the initial solidification environmental of three-phase point is degradation, all of these are not conducive to obtain good surface quality billet. With the conditions of appropriate meniscus height, frequency of 2500Hz, power of 30kW, the meniscus height is almost as same as that under high-frequency soft contact electromagnetic continuous casting with frequency of 20.5 kHz and power of 67.2kW, a significant energy is saved. When the position of free surface is in the center height of the coil, the meniscus height is maximum. When the free surface paralleled the top and bottom of induction coil, meniscus fluctuations is relatively stable. When the position of free surface was more closed to the induction coil centers, the meniscus fluctuations become more intense. In the casting process, under the appropriate power, the free surface should be controlled near the center under partial of coils in order to ensure the effectiveness of soft contact and energy-saving. With the frequency increasing, the meniscus height decreases almost linear relationship, the meniscus fluctuation reduce, the attenuation range of fluctuation variance reduce. Considered the distribution of magnetic induction intensity, meniscus height and fluctuations, the frequency of 2500Hz is suggested to the appropriate parameter of intermediate frequency soft contact electromagnetic continuous casting...