| Continuous casting with electromagnetic stirring (EMS) is an important technology to improve the quality of ingots. It controls solidification process of ingots with the aid of induced electromagnetic (EM) force to promote the movement of molten steel. EMS can promote the mixing of cool and hot molten steel on the solidification front, decrease temprature gradient, improve solidification structure and cleanliness of molten steel, expand isometric zone, and reduce segregation, central porosity and shrinkage, etc. The basic theory study of EMS mainly includes the theoretical analysis and the numerical simulation. It is necessary to use numerical simulation as theoretical analysis is so difficult.This paper bases on a factory continuous casting mold with EMS of round billet. EM field model of round billet mold with EMS was established by using ANSYS software. We analysed the distribution of EM field and the influence mechanism of process parameters. The accuracy of the model was verified through the measured data. The results are as followed:(1) The established model is reasonable.(2) The values of magnetic induction intensity reach the maximum at the center of the stirrer and become smaller gradually towards two ends in the axis direction, but increase from the center to the edge in the horizontal direction. Tangential EM force reaches its maximum at the center of the stirrer and increases at the exit of mold as the absence of the shielding effect of copper tubes. Tangential EM force increases as the distance from the center of the circle increases in the horizontal direction.(3) The magnetic induction intensity has linear relationship with the current intensity. At the frequency of 2.5Hz, as the current intensity increases by 100A, the magnetic induction intensity at the center of stirrer increases by about 12mT. Tangential EM force increases like parabolic orbit approximately as the current intensity increases.(4) There is a linear relationship between the magnetic induction intensity and the current frequency. At the current intensity of 400A, as the current frequency increases by 1Hz the magnetic induction intensity at the center of stirrer decreases by about 6.8mT. As the frequency increases, the tangential EM force increases firstly and then decreases, which reaches its maximum at 8.0Hz.The flow field calculation model of round billet mold with EMS was established by using CFX, adding the mean values of the EM force obtained from simulation of EM field to the momentum equation. And the distribution of flow field and the impact mechanism of different process parameters were received. The results show that:(1) Without EMS, the flow velocity of molten steel is very small in horizontal direction, and the maximum velocity in the central plane of the stirrer is 0.010m/s. A simple flow pattern is formed in axis direction.(2) With EMS, molten steel flows like swirling in horizontal direction. Tangential velocity reaches the maximum at 135mm from the central position, and then decreases rapidly due to the surface effect. When the current intensity is 400A and current frequency is 2.5Hz, the tangential velocity reaches its maximum of 0.560m/s. A secondary flow pattern is formed in the axis direction.(3) As the current intensity increases, the tangential velocity increases, the position of the mainstream zone rises, and the scope of steel at the bottom of the mainstream zone enlarges. With the increase of current frequency, the changing rule of the flow field is the same as the increase of the current intensity. Drawing speed has less impact on the tangential velocity, but the initial speed of molten steel increases as the drawing speed increases, the position of the mainstream zone declines and the scope of steel at the bottom of the mainstream zone narrows down.According to the scope of reasonable parameters obtained by numerical simulation, on the basis of the existing process conditions, optimization experiment was carried out in view of the process parameters of M-EMS. We tested the macroquality of round billet under different experimental parameters.The results are as followed:(1) The current intensity affects the center equiaxial crystal more significantly than the current frequency.The center equiaxial crystal has been basically saturated and it reaches the maximum of 38.4% at the current intensity of 380A. At this time, it makes no sense to increase the current intensity to improve the equiaxial crystal of round billet.(2) Considering the center equiaxial crystal and macroquality results of different parameters, the optimal process parameters of M-EMS were obtained. The current intensity is 400A and the current frequency is 2Hz. Under the optimal process parameters, the equiaxial crystal is 38.2% and the average central porosity is 0.95. |
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