Research On Enhanced Cooling Mechanism Of Pressure During The Solidification Process Of High Quality Special Steels

As one of important parameters,pressure can significantly improve the heat transfer conditions at the ingot and mold interface,it has a great influence on the solidification quality and mechanical properties of ingot.However,the current research on pressurized solidification is mostly concentrated on the field of non-ferrous metallurgy,especially the effect of pressure on interfacial heat transfer,which is rarely reported in the field of iron and steel metallurgy.In this paper,the temperature and air gap changes in the ingot solidification process are measured by the temperature and air gap measurement experiments,and thermal stress calculation model of pressurized solidification was built based on ProCAST software.The effects of solidification pressure on the interfacial air gap and heat transfer coefficient are studied.The main contents and conclusions of this paper are induced as follows:(1)An experimental device for measuring temperature and air gap under pressure conditions is designed,and the temperature and air gap in the smelting process of 18Cr18Mn high nitrogen austenitic stainless steel under 0.15MPa is measured,the results show that the maximum air gap is 0.75mm.Comparing the temperature measurement curves under 0.15MPa,1MPa and 2MPa,it is found that the increase of solidification pressure can significantly increase the cooling rate of the ingot.(2)Based on the stress analysis of ingot under pressurized solidification,thermal stress model of pressurized solidification was built by ProCAST software,and the thermal stress model of pressurized solidification was verified combined with experimental data.The result shows that the simulation results of temperature and air gap are in good agreement with the experiment under the condition of the interface thermal resistance.Therefore,this model has good accuracy and reliability,and the physical parameters and initial conditions are reasonable,the simulation calculation of the temperature and stress field under pressure can be carried out.(3)The simulation calculation of interfacial air gap under different process parameters(solidification pressure,casting temperature of molten steel,preheating temperature of mold and protection atmosphere)is carried out by thermal stress model of pressurized solidification.The result show that the air gap decreases with the increase of solidification pressures,the pressure can effectively improve the interface heat transfer conditions and increase the interface heat transfer coefficient.The higher casting temperature and the thermal conductivity of the protective gas,the larger of the gas gap between the ingot and the mold.The increase of the preheating temperature of the mold will lead to the decrease of the temperature gradient and the thermal stress in the ingot,and the air gap in the interface decreases.(4)Based on the experimental and simulated data on the change of heat transfer coefficient and air gap with pressure in the solidification interface,the influence of solidification pressure increase on the interface heat transfer coefficient is mainly reflected in the formation of air gap.The inverse calculation results of the interface heat transfer coefficient show that the interfacial heat transfer coefficient tends to increase with pressure.The theoretical and simulated results of air gap under different solidification pressure also show that the increase of interfacial air gap is obviously reduced by the increase of solidification pressure.(5)Combining the mathematical model for the variation of the gas thermal conductivity with pressure,the variation of thermal conductivity of different gases with solidification pressure is analyzed.The result shows that the influence of pressure on the thermal conductivity of gas is obvious at high temperature.The pressure intensive cooling is mainly reflected in a large reduction in the interfacial air gap,increasing the thermal conductivity of the gas and the interfacial heat transfer coefficient.Morover,the increase of the solidification pressure also increases the radiation heat transfer coefficient to a certain extent,and the main heat transfer mode is gas conduction heat transfer after air gap generated.