Mathematical Modeling Of Solidification During Electroslag Remelting Process And Development Of New Slags

Electroslag remelting (ESR) has been used for some special materials since its birth due to the ingot quality of sound structure, uniform compositions, surface smoothness and high yield. For a long time, the process parameters were determined by experience, which has less theoretical guidance. A lot of mathematical models had been established for describing the phenomenon of ESR process and explored the optimum process, but still many problems should be studied and solved.The power consumption of ESR process is very high when many existed slags are used. At present, the power consumption is 1500～1600kWh/t steel for ESR cold roll steel in a domestic plant. At the same time, hydrogen pick-up during ESR process is very high for hydrogen sensitive steel; the further long time heat treatment is needed for decreasing the hydrogen content. The content of some segregation phase occurs when some prone segregation alloy is produced by ESR, which needs to be controlled. The existing problems for ESR process have been studied aiming at improving ingot quality, decreasing power consumption and the content of gas and harmful phases in our present work.Slag L1, L2 and L3, in which the oxide components were added to decrease the electric conductivity have been designed, in order to decrease the high power consumption when slag LO containing 70%CaF2 and 30%Al2O3 was used in this plant. It indicates from the results of melting point, viscosity, basicity, density and electric conductivity that the designed slags can meet the requirement of ESR process. At the same time the designed slags have low electric conductivity, which is advantaged for decreasing the power consumption.The experimental results indicate that slag LO has the lowest hydrogen permeability, which is coincident with the low hydrogen content in steel after remelting with slag LO in many literatures. The measured value of hydrogen permeability for slag L0 is 0.48×10-6 mol·cm-1·min-1 in this work. The hydrogen permeability is 0.98×10-6mol·cm-1·min-1 for slag L2, which is lower than slag L1 and L3, so it is a promising slag for remelting some special steels. There are many influence factors on hydrogen permeability. Firstly, optical basicity has great effect on hydrogen permeability and the higher optical basicity is the higher hydrogen permeability for general trend. Secondly, though CaO has bigger water capacity, the higher CaO content in slag attributes to increasing hydrogen content in steel. Whereas, Al2O3 is advantaged for preventing hydrogen permeation for its low water capacity and exists as acid oxide. Research shows that the higher the parameter EH is the higher hydrogen permeability, which is put forward for an evaluation parameter of hydrogen permeability of slag in this paper.The ESR experiment results with all kinds of slag studied in this paper indicate that hydrogen content is highest at the beginning of the remelting process, which is controlled by water in slag, hydrogen permeability of slag, hydrogen content in electrode and atmospheric moisture. Later the hydrogen content in steel is kept at a lower content, which is influenced by hydrogen permeability of slag, hydrogen content in electrode and atmospheric moisture here. The increase of hydrogen is low after remelting with the lowest hydrogen permeability of slag L0. The increase of hydrogen is also low after remelting with slag L2. However, the hydrogen content is higher after remelting with slag L1 and L3. It has been found that not only the hydrogen content is lowest at the beginning of the ESR process but also the increase of hydrogen is lowest when protective atmosphere and premelted slag are adopted. Water capacity and atmospheric moisture have little influence on the process in this case. And then the inclusions in remelted ingots with diferent slags were analyzed. Results show that the amounts, of inclusions in steel in the case of slags LO and L3 are less than that with slags L1 and L2. But a lot of nitride inclusions were found in steel remelted with slags LI and L3, which resulted from their high hydrogen permeability.The power consumption test results at industrial ESR show that the slag L1 and L0 has almost the same power consumption, and slag L2 can decrease the power consumption with 200kWh/t steel. Furthermore the products passing rate is higher than before using the new slag, which will have huge economic benefits.A mathematical model, based on electromagnetic field equation, fluid flow equation, and heat transfer equation, was established for the simulation of the electroslag remelting process. The distribution of temperature field was obtained by solving this model. The relationship between the local solidification time and the interdendritic spacing during the ingot solidification process was established, which has been regarded as a criterion for the evaluation of the quality of crystallization. The results indicate that the temperature of slag center is above 1800℃. For a crucible of 950 mm in diameter, the local solidification time (LST) is more than 2300s at the center of the ingot with the longest secondary interdendritic spacing, which is about 500μm. Whereas LST is only 600s at the edge of the ingot according to the calculated results, which is about 200μm for corresponding secondary interdendritic spacing. Secondary interdendritic spacing increases from the edge to center of ESR ingot, and therefore the secondary interdendritic spacing can be used to estimate the ingot quality.The power supply parameters was calculated using the proofed model for the crucible ofΦ130mm in diameter and the experiment was carried out using alloy IN718 in laboratory. It was found after experiment that the non-metallic inclusions content decrease after ESR process and the alloy ingot was sound structure, uniform compositions, surface smoothness and no macroscopic defect with the rational power supply parameters. The content of Laves phase and the extent of element segregation decrease. At the same time the optimal melting rate is 66kg/h forΦ130mm crucible by the experiment analysis. The corresponding LST in ingot center is 375s, and the solidification rate of the ingot is 10.1mm/min.The further experiment was carried out in laboratory using Cr5 cold roll steel. The power supply was calculated by model for different slags. The analysis results indicate that the carbide content in steel is low using the parameters ascertained by model. The results have proved that model can provide theoretical basis for establishing the ESR power supply parameters.The ESR experiment in the industrial scale shows that the predicted shape of molten pool is in agreement with the measured results after the model has been revised, so the model is accurate. It has been found that macrosegregation of most elements in ESR ingot can be controlled at a range of 0.95-1.05 after the Cr5 cold roll steel had been analyzed. The gas content in steel is low, which meet the requirements of steel. Secondary interdendritic spacing gradually increases from edge to the center of the ingot and the largest one measured is 557μm. The content of carbide phase increases form the edge to the center of the ingot and the graininess gradually augments, which has great relationship with the increase of interdendritic spacing. The carbide existing in Cr5 ESR ingot is mainly M7C3 and MC type analyzed by SEM.Some defects had been found in the ingot, which have great relationship with the high melting rate and indicate that the power supply is not enough reasonable. Further power supply have been calculated by model and the capacitity need to be confirmed.