Multi-Physics Field In ESR System With Pipe Electrode And A New Hot-Topping Technique In The Solidification Control Of Large Ingot

Electro-slag remelting(ESR)is a special method of melting metals and alloys,which can integrate the refinement and solidification of the metal,and improve the purity and crystal structure of the metal.However,with the rapid development of industry,the requirement for the ingot of high quality is rising,the product of traditional electro-slag remelting has some defects like segregation,coarse microstructure,shrinkage cavity and porosity and so on,to solve this problems,this article has presented the electro-slag remelting with pipe electrode,calculated the multi-physics field distribution of this system with the numerical simulation method,and compared with the traditional electro-slag remelting with solid electrode,it found that the product of the electro-slag remelting with pipe electrode had higher quality,besides,this article also made an optimization analysis for different sizes of pipe electrodes to provide effective prediction and theory support for the practical application,the main conclusions of this part:(1)Through the comparison of pipe electrode and solid electrode with the same cross section current passed in consumable electrode and loading conditions in electro-slag remelting system,it found that the current had different divergence patterns at the bottom of the consumable electrode due to the different shapes of consumable electrodes,this made a big difference in the distribution of joule heat power density,the joule heat power density of electro-slag remelting system with pipe electrode reduced in the center of slag pool and increased near the external surface of slag pool compared with the solid electrode,the maximum appeared at the corner under the consumable electrode in slag was lower.(2)The biggest difference of the temperature distribution between pipe electrode and solid electrode with the same boundary conditions in electro-slag remelting system was mainly reflected in the slag pool,compared with electro-slag remelting with solid electrode,the outstanding advantage of the electro-slag remelting with pipe electrode was that the temperature of the slag pool was more uniform,this could lead to a more shallow metal molten pool and create an ingot under directional solidification.(3)This article has made an optimization analysis for different sizes of pipe electrodes from the perspectives of joule heat,temperature and metal molten pool,it found that in these models,when the inner radius was 0.25m,the ratio of inner radius to outer radius was 0.679,the total joule heat of the slag pool and the power of the whole system was the lowest,besides,the temperature difference of the slag pool was the lowest,the temperature of slag was the most uniform,and the temperature of this model met the demands of the melting of electrode,the metal molten pool was shallow and flat relatively,so the quality of the ingot was the best.Another part of this article is the electro-slag hot-topping(ESHT),it is an application of the electro-slag remelting that belongs to electro-slag metallurgy technology.A new device of electro-slag hot-topping with two series-connected electrodes has been designed to solve the non-uniform temperature problem in the hot top of the technology of electro-slag hot-topping during the solidification of large ingot at present,two series-connected graphite electrodes powered by alternating current with one solid inside and another pipe outside were inserted in slag pool at the top of the ingot,current passed the central electrode,slag pool,pipe electrode and formed a closed loop with external short net,only a small part went into the molten steel,so the high temperature heat source generated in the slag pool could be used to heat the molten steel,this technology could make the metal in riser keep liquid during the solidification of the ingot until the solidification was completed,and reduce some defects like shrinkage cavity and porosity existed in the center of the ingot,and improve the quality of ingot significantly.In addition,numerical simulation method was also adopted to develop a model to analyze the distribution of the multi-physics filed during the solidification of the ingot with ESHT,and this analysis provided effective prediction and theory support for the technology,the main conclusions of this part:(1)In this system:the current density of the two electrodes was uniform with similar size,the maximum appeared at the corner under the consumable electrode in slag.The clockwise magnetic field was mainly distributed near the slag pool and electrode,the magnetic flux density increased at first and then decreased along the radial direction from the central axis of the electrode to the edge,the maximum appeared at the external surface of the central electrode.The electromagnetic force was radial in two electrodes but the direction was opposite influenced by the current density and magnetic flux density,the electromagnetic force had an axial component in slag pool due to the radial component of the current density.The joule heat mainly distributed in slag pool,the maximum appeared at the corner under the consumable electrode in slag.(2)In this system:the temperature of the slag was higher,high temperature region was at the bottom of the central electrode,the molten steel under slag solidified affected by the water in the mold,the temperature near the wall and the base was lower so the molten steel of this region solidified first,the molten steel in the riser kept liquid due to the slag heating.The high temperature heat source generated in the slag pool could heat the molten steel in the riser and make molten steel temperature increased significantly by compared with no heating.(3)It found that the most important factor affected electro-slag heating was the heat source distribution pattern in the slag pool through the research on the slag thickness and the electrode immersion depth,the more uniform the joule heat distributed,the higher the temperature of the molten steel in the riser was heated,the better the device was.In these models,when the slag thickness was 0.1 m,the electrode immersion depth was 0.05m,the distribution of the joule heat in the slag was the most uniform,the average temperature of the slag was the highest,the temperature of the molten steel in the riser heated was the highest,the effect of the device was the best.