Mathematical Simulation And Experimental Research On Electroslag Remelting Process Of Hollow Ingot

With the rapid development of electric power,energy,petrochemical engineering,storage and transportation,the need for large cylindrical forgings are increasing with the inevitable quality and quantity demands.The solid ingot is used as raw materials in the traditional production of cylinder forgings.The hollow forging is manufactured from solid ingot by upsetting and punching at first,and then forging and machining.However,using the hollow ingot as raw materials for forging,which can save upsetting and punching process,will reduce the forging times and forging crack,improve forging yield,reduce the ingot weight and make full use of the equipment capacity.In recent years,the market demands of hollow ingots are increasing with the improvement of the localization in the field of petroleum hydrocracking and power equipments.With the development of electroslag remelting(ESR)technology,ESR technology becomes a new method for producing hollow ingots.This new method can avoid the quality problems of the hollow forgings seriously caused by crack and segregation defects during the traditional hollow ingot production process.In this paper,this method was applied to manufacture Mn18Cr18N hollow ingot for retaining ring forging piece,which can avoid cracking of the retaining rings steel during forging forming process.A new way for the retaining ring steel production is developed,which can meet the demand for high quality and high performance of raw materials.Because the hollow ESR withdrawal process is different from traditional ESR process,the new and suitable slag systems have been developed for withdrawal process in this paper.Four kinds of slags(S1～S4)were designed and compared with two commonly used slags(S5,S6),by testing and calculating the various physical properties.The tested results show that the melting temperature of designed slags are lower than that of existed slags and the viscosity of designed slag are quite stable with the temperature change at high temperature,and the melting temperature and viscosity stability of slag S4 are the lowest and best,respectively.The calculated results demonstrate that,for slag S4,the higher density and basicity can contribute to remove inclusions and sulfur in steel,the lowest conductivity can reduce the power consumption and improve productivity during ESR process.It is considered that slag S4 is the most suitable for ESR withdrawal process that was validated by the remelting experiments.In this paper,the mathematical model of multi-electrode hollow ESR system has been established.Based on Maxwell equations,Navier-Stokes equation,continuity equation and heat transfer equation,using the sequential coupling,the physical field distribution in the slag bath and hollow ingot can be simulated.At first,the mathematical model with the outer and inner mold size of Φ300/100 mm,steel grade of Mn18Cr18N has been established,which is the same as the laboratory scale ESR equipment and verified by comparing the results of simulation calculation with the measured depth of metal pool.The optimized technological parameters have been obtained by simulating the effect of the different process parameters on the shape of the metal pool and verified by the experiments.Secondly,the mathematical model with the outer and inner mold size of Φ 650/450 mm has been established and verified,which is the same as the industrial scale ESR equipment.At last,the mathematical model with the outer and inner mold size of Φ 900/500 mm has been established based on the second model.The influence of different process parameters,which are the depth of slag pool,electrode insert depth,electrode arrangement and mould power supply mode,on slag temperature and velocity distribution and the characteristics of steel ingot solidification have been calculated.The obtained optimal process parameters also have been confirmed in industrial tests.A comprehensive mathematical model of the solidification structure during the process of hollow ESR was established by using the moving boundary method and the coupled technology CAFE method(cellular automaton(CA)-finite element method(FE)).At first,the model of ESR ZG06Crl3Ni4Mo solid ingot was established and verified by the experimental results.Then the model of ESR Mn18Cr18N hollow ingot was established,and the simulation results of the metal pool shape,dendrite morphology and the secondary dendrite spacing(SDAS)were in good agreement with the experimental results.Furthermore,the effect of different melting velocity,mold cooling conditions and bottom cooling conditions on the hollow ingot solidification structure have been revealed,which can provide a theoretical basis for optimizing technology parameters.The most suitable slag and optimal process parameters for ESR of Mn18Cr18N hollow ingot with Φ0300/100mm have been validated by lab scale experiments.The effect of matching between the power supply and melt velocity on surface quality have been investigated.The obersevation of macrostructure and microstructure can illustrate that Mn18Cr18N hollow ingot with Φ300/100 mm has the uniform density and fine grain without porosity and shrinkage cavity,which are in agreement with the results obtained by simuation of solidification structure.It was indicated that using slag S4 can get removing of inclusion and sulfur greatly,the less burning loss of main elements,the finer grains and the higher compactness of structure through measuring of chemical composition,inclusion and SDAS.The Mn18Cr18N retaining ring has been manufactured successfully by using hollow ESR ingot in lab scale,which has perfect processing plasticity,nice malleability at high temperature to reduce the forging cracks.Finally it can be used to make a retaining ring with high strength and high ductility for practical application.The industrial tests for ESR of large hollow ingots have been carried out on the basis of the above mathematical simulations and laboratory tests.At first,the test for ESR of hollow ingot with size of Φ650/450 mm have been made to investigate the effect of different slags on the surface quality of ingot in order to choose more suitable slag for withdrawal process.Secondly,the industrial tests for ESR of hollow ingot with the size of Φ 900/500 mm have been performed by operating the optimized process parameters.The surface of obtained hollow ingot was smooth and free from obvious defects.Finally,a perfect hollow ESR ingot was remelted successfully with the excellent surface and internal quality.The current supply mold(CSM)is used for hollow ESR experiment to compare the effect of different power supply mode on the surface quality of ingot.The whole ESR process was stable without the leakage of slag and liquid steel.There was a deep slag pit around the circle of ingot surface in the case of non-CSM and but no obvious defect in the junction where the operation of electrode exchange was performed in the case of CSM.