| Obtaining ingots with high cleanliness and excellent solidification structure is an important objective that metallurgists always pursue.Electroslag remelting(ESR)process technology is a simple and effective refining process.When the ingots are refined by the ESR process,the content of inclusions can be significantly reduced and sounder structure can be obtained.However,as the size of ingot increases(e.g.,the ingot diameter is more than 1 m),the cooling condition is deteriorated during the solidification process,which may cause problems such as coarse carbides,excessive inclusions,severe macrosegregation and so on.Sometimes,the large ingot may be scrapped for the serious defects,causing heavy losses or safety hazard.In fact,with the increase of the electroslag ingot size,the solidification process of the electroslag ingot will encounter all the problems happening in the conventional solidification process of the large ingot.At present,the solution to the solidification defects of large ingots or electroslag ingots is extremely scarce.Considering the alternating current used in the ESR process can reach up to thousands or even ten thousands amperes,if an external static magnetic field is applied,the forceful alternating Lorentz force can be generated in the liquid film attaching the consumable electrode tip,slag pool and metal pool during the ESR process,which will have a significant impact on the refining process and solidification process.Based on this,my research group has proposed a new concept which was called magnetically controlled electroslag remelting(MC-ESR)technology.Comparing with the alternating magnetic field,there is no skin effect generated by the static magnetic field imposed during the ESR process.So,the MC-ESR technology has a great potential to solve the solidification defects of large electroslag ingots and improve the refining efficiency.However,due to the high temperature and invisible of the ESR process,the influences of the magnetic field on the transfer behaviors in the liquid film,slag pool and metal pool are still unclear.Therefore,the physical and numerical simulation method combining with small-scale experiments were used to investigate the transfer behaviors during the MC-ESR process.The influences of the static magnetic field on the electromagnetic field,flow field and temperature field in liquid film,slag pool,and metal pool were studied.The migration behavior of inclusion was simulated and the solidification process of the electroslag ingot was studied.The research content of this thesis mainly includes the following aspects:① The physical visualized simulation equipment was built to study the influences of different intensities of transverse static magnetic field(TSMF)and remelting current on the formation and dripping process of droplet during the MC-ESR process with the current frequency of 50 Hz.When the external TSMF of 0.7 T was applied,a special phenomenon was discovered:the liquid neck connecting the electrode tip with the droplet would be smashed into lots of smaller dorplets.The smashing effect would be improved with fhe enlarging of the external TSMF under the condition of a constant remelting current.② Based on the established physical visualized simulation equipment,the influences of different remelting current frequencies on the droplet evolution process under the consumable electrode tip were investigated during the MC-ESR process with a variable frequency power supply.The results shew that the variation of the current frequencies ranging from 5 to 500 Hz had little influence on the droplet evolution process during the ESR simulation with no external magnetic field.When the MC-ESR process adopting the TSMF of 0.7 T,a lower current frequency was more beneficial to improve the breakup effect happening on the droplet neck.The average size of the main droplets could be reduced and the droplets were not concentratedly dripped into the center place of metal pool under the action of the Lorentz force with low frequency.The breakup effect and the refinement of dorplets would increase the interfacial area and improve the dynamic condition of inclusion removal.③ The three-dimensional numerical model of droplet evolution during the MC-ESR process was established.The model was coupled with the alternating remelting current,the external magnetic field and the two-phase flow.The influences of the magnetic field with different acting direction on the droplet evolution were investigated,the simulation results could be well matched with the experimental results.Numerical simulation results confirmed that the shape of liquid droplet neck would be changed from a column into a flat strip by the effect of electromagnetic vibration(EMV),and then the whole droplet neck could be smashed into a lot of smaller droplets from its center position during the MC-ESR adopting a proper TSMF.Meanwhile,the droplets will not drip into the metal pool in the center area,but in a dispersed manner.The scattered droplets will be beneficial to form a shallower metal pool.④ The numerical model investigating the migration behaviors of the inclusion in the liquid film under the comsumable electrode tip was established.The simulation results shew that when the velocity gradient existing in the liquid film was large enough,the generation of Saffman force could make the inclusion overcome its Buoyancy force and migrate from the position near unmelted electrode to the melt/slag interface within a short distance and time.As the increase of velocity gradient in the liquid film,the inclusion particle with the same size needed shorter distance and time to reach the melt/slag interface.When the velocity gradient in the liquid film layer under the consumable electrode tip is large enough,the larger the size of the inclusion particle,the shorter distance and time for the inclusion particle to reach the melt/slag interface,which meant the inclusion particle could be more easily removed.⑤ The MC-ESR experiments on PWA1483 nickel-base superalloy were carried out.The consumable electrode with the diameter of 60 mm was remelted in a water-cooled copper mold(Φ100×500mm).The experimental results shew that when the ingot refined by the MC-ESR adopting the TSMF of 0.05T,the equiaxed grain zone in the core of the ingot was enlarged,the grain size in the equiaxed grain zone was significantly decreased,and the yield strength and tensile strength were optimal.Besides,the numerical model investigating the solidification process based on the conditions of the actual MC-ESR experiment was established.By the method of equivalent treatment,the effects of different intensities of TSMF on the solidification process were calculated.The simulation results shew that along with the increase of the TSMF,the temperature gradient along the longitudinal direction of ingot was decreased,the depth of metal pool became shallower,the macrostructure of ingot was better refined,and the equiaxed grain zone was enlarged.In conclusion,the results in this thesis of physical simulations,numerical simulations and experiments shew that the MC-ESR process adopting a proper TSMF acting with the remelting current could generate the forceful EMV,which would enhance the momentum of metal film under the comsumable electrode tip and made the inclusion particle reach the melt/slag interface more easily.The EMV could also smash the droplet neck into lots of smaller dorplets and refine the main droplet size,and make the droplets dispersed drip into the metal pool.Meanwhile,the effect of EMV could strengthen the convection flow of molten metal in the metal pool,thereby improving the solidification conditions of metal pool.These are all beneficial to obtain the ingots with high purity and excellent quality.This study provides the legible theoretical and practical basis for further development and application of MC-ESR technology in the future. |
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