| The mechanism of electromagnetic stirring(EMS) in the continuous casting bloom and slab which improves the strand quality is intensive fluid flow and heat/mass transfer during solidification process by means of electromagnetic force with non-contact stirring. It is an effective technique to improve the inner and surface quality of cast products, such as reducing inclusions and pinholes, eliminating center-line segregation and porosity, expanding equiaxed zone. Therefore, it is the first choice when cast large-size bloom, slab and the steel which need high quality or appear bad quality easily.In this paper, the method of numerical simulation and experiments were used to research the flow behavior of mold electromagnetic stirring(M-EMS). First, the three-phase, sextupole rotating EMS in laboratory was chosen as the research objects. The distribution of the magnetic induction intensity and electromagnetic force for with or without mold, with or without steel, with different winding type and relationship between EMS structure size and magnetic induction intensity were all simulated. And the treatment of low Renolds number k-ε model and wall functions which used in near wall were compared.The copper mold has great effect on magnetic field, the magnetic induction intensity becomes samller with copper mold compared to without copper mold. The magnetic induction intensity with melt steel in mold is a little larger than that without melt steel, but the difference is so small that the testing results can be used to verify the simulation results.Based on above results, the rule of M-EMS in the large-size strand (82B high carbon steel, GCr15bearing steel) during the casting process in two domestic steel plants was investigated by numerical simulation and experiments. The research mainly focuses on the characteristics of electromagnetic field, flow field and grasping the optimal parameters.In order to verify the correctness of numerical simulation, the magnetic induction intensity with M-EMS was measured in the two steel industries. The comparison shows that the calculated results confirm the experimental data. An experimental investigation was also carried out in an electromagneticaily stirred Woods metal system. The flow state was measured with various parameters and position using high-speed video photography, which can verify the simulation results in steel plants. Finally, the industrial plant trials of M-EMS on-line application were performed.First, the magnetic field of different bloom during continuous casting with M-EMS was solved by the finite element method. Then, the electromagnetic volume force of magnetic field calculation as source term added to the Navier-Stokes momentum equation, the coupling calculation of flow field was solved by the finite volume method, the low Reynolds number k-ε model was adopted to model flow near the wall. The influence of M-EMS current, frequency and other parameters on distribution of magnetic field and flow field were researched. Full coupling was chosen to predict the effect of the particles on the melt steel, which also affects the flow field.The results are as follows:I.Electromagnetic field resultsThe results show that the change of coil width can’t affect the distribution of magnetic induction intensity and electromagnetic force both on the center plane of steel and the center line of outer surface. The magnetic induction intensity and electromagnetic force reach peak in EMS center and the maximum value increases as the coil width increases. The different proportion of pole and yoke can effect on the distribution of magnetic induction intensity and electromagnetic force on the core. The different proportion of pole and yoke has little effect on the distribution of magnetic induction intensity and electromagnetic force in the center line of outer surface. The results provide the basis information on how to design EMS, define the structure size of EMS and optimize the M-EMS effect. The comparison of low Renolds number k-ε model and wall functions showed that using low Renolds number k-s model can simulate the near wall better.Ⅱ.For φ250mm82B high carbon steel(1) Without M-EMS, distribution of velocity is along the radial direction, velocity is small, the maximum velocity locates in center of bloom, only0.3m/s; with M-EMS, velocity is along the tangential direction, the maximum velocity can be up to0.75m/s of current and frequency are480A,3Hz, the maximum velocity located on the edge of bloom, and does vortex-like flow.(2) At the same current, magnetic induction intensity decreases with the increase of frequency, electromagnetic force and velocity increase with the increase. With the increase of frequency, the maximum magnetic induction intensity and electromagnetic force variation rate become more and more smaller; while the velocity decreases at first and then increases as the frequency increases along the casting direction. The magnetic induction intensity and electromagnetic force decrease with frequency increases in the radial of EMS, the influence of frequency on tangential velocity is very little.(3) At the same frequency, magnetic induction intensity, electromagnetic force and velocity increase with the current. The influence zone of magnetic field become larger, the turbulence zone also becomes more strenuous. Expecially on the free surface, the proper turbulence benefits the uniform steel temperature.(4) Trial results show that the quality defects of82B high carbon steel billet can be significantly improved with M-EMS. The zone of central pipe decreases. With the increase of current, equiaxed zone ratio increases. Unidirectional stirring can promoted columnar dendrites to transit equiaxed dendrites effectively, which can improve center equiaxed zone ratio.(5) Considered the results of numerical simulation and experiments, the optimized EMS parameters of φ250mm bloom are480A and3Hz for the stirring current and frequency, respectively.III.For470mm×350mm GCr15bearing steel(1) With the influence of caster arc, the two back flows are not symmetric, outside back flow is larger than the inner. Different to round bloom, the velocity is very small in corner with the shape influence.(2) Fixing the other parameters, magnetic induction intensity, electromagnetic force, velocity, turbulence kinetic energy and turbulence kinetic energy disspation all increase with the current. But the velocity decreases with current of600A in casting direction.(3) Magnetic induction intensity decreases with the increases of frequency, electromagnetic force, velocity, turbulence kinetic energy and turbulence kinetic energy disspation increase with the frequency increase. With the increase of frequency, the variation of maximum magnetic induction intensity, electromagnetic force and velocity decreases. (4) The maximum velocity in center cross-section of EMS has direct proportion relation to current, quadratic relation to frequency.(5) With the increase of mold depth, the shielding effect strengthen, magnetic induction intensity, electromagnetic force, velocity decrease, the position of maximum magnetic induction intensity postpone gradually; the position of maximum electromagnetic force does not change; the range of magnetic field influence zone changes little.(6) With M-EMS, inclusions do rotating motion with molten steel, and with the current increase, more and more inclusion particles accumulate in the upper zone of mold, so the removal rate of inclusion particles increase. With the increase of frequency, the amount of inclusiong to upper zone of mold also increases, but the zone of rotating moves down gradually.(7) For470mm×350mm bloom during continuous casting with M-EMS, the current, frequency and mold depth of550A,2Hz and35mm can play well stirring effect. |
PDF Full Text Request