| Defects like central gravitational and dendritic segregation in the process of continuous casting can be improved by final electromagnetic stirring (F-EMS) effectively. This subject is in the light of the FEMS installation of Baoshan Iron and Steel Company Limited. With the New-type multi-functional electromagnetic stirring installation, the process of FEMS of the continuous casting was simulated in the experiment conditions. Low melting alloys Sn-Sb and Sn-Pb were selected in this paper which were stirred on with spiral magnetic field (SPF), with comparison of alloys solidified in the travelling magnetic field (TMF). Effect and mechanism of action under SMF on macrosegregation improvement, microstructure refinement were analysed at different excitation current and frequency which was expected to provide a theoretical basis for the practical application of a novel stirring method in F-EMS. Sn was selected in this experiment to be the matrix metal, and the composition was designed to Sn-11%Sb and Sn-20%Pb. Sn-11%Sb was selected to analyse the effect of SMF and TMF on the gravity segregation of the ingot as well as Sn-20%Pb was selected to analyse the effect of SMF, TMF and anti spiral magnetic field on the micro structure of the ingot.Sn-11%Sb was selected with the stirring methods of TMF and SMF, frequency of lOHz and excitation current of25A,50A,75A,100A,125A. Ability on improving the gravity segregation becomes better with the increasing of the excitation current for both of the two magnetic field. Effect of gravity segregation improvement reach a optimal value with1.67vol%for the content of β phase difference between upper and lower part of the ingot when the excitation is125A under TMF stirring;(3phase difference is0.56vol.%when the excitation is100A under SMF stirring. Difference between upper and lower part of the ingot is only0.08wt.%for quality score of Sb analysed by XRF under SMF which means the gravity segregation of the ingot is substantially eliminated. It could be found that, from the experimental analysis, effect of SMF on gravity segregation improvement is more excellent than TMF.In order to simulate the process of FEMS in the continuous casting, relationship between the solidified shell thickness and solidified time was measured in which6.5min was confirmed to be the beginning stirring time after pouring and the shell thickness was about19mm with a solidification rate of70%. When the excitation current increase from75A to150A, the minimum value of the average grain size unde TMF is154μm when the excitation current is150A and comparatively, the minimum value of the average grain size under SMF is 133μ m when the excitation current is125A. An optimal value of frequency can also be found for both SMF and TMF when changing the stirring frequency. The minimum of the average grain size is153μm when the frequency is lOHz and13Hz under TMF while the average grain size is133μm when the frequency is lOHz under SMF. From the analysis it could be found that, compare to TMF, a more intense convection could occur in the melt when the rotating magnetic field (RMF) and travelling magnetic field (TMF) superposed with each other (SMF) which can cause the fracture of dendrites more easily, therefore SMF shows a more excellent effect on solidification structure refinement with the optimal stirring parameters of125A and10Hz. Furthermore, solidification time could be deferred under the intense convection and more induction heat will be brought in, for this reason, a higher excitation current and frequency over the optimal value will lead to a coarse microstructure and the growth of dendrites. It could be found that when the melt was stirred on with the anti spiral magnetic field with an upward direction travelling wave, a worse effect would be showed than SMF on the ability of dendrites fracture and microstructure refinement and the sinkhole could appear easily due to the gas which couldn’t be let out timely. |
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