Effect Of Air Gap At The Interface Of Mould-ingot On The Comprehensive Interfacial Heat Transfer Coefficient

The interfacial heat transfer during the cooling and solidification of the ingot is very complicated.The interfacial heat transfer coefficient at the mould-ingot interfacial is the core parameter that affects the solidification process.It is affected by many factors including pouring temperature,Preheating temperature of mould,thermal properties of ingot and shape of ingot.During the cooling and solidification process,due to the cooling shrinkage of the ingot and the thermal expansion of the mould,air gap will be generated between the two interfacial.Due to the formation of the air gap,the thermal resistance between the mould and the ingot becomes the main factor affecting the interfacial heat transfer coefficient.However,the mechanism of air gap formation during the solidification process of steel ingots and the influence of air gaps on the interfacial heat transfer coefficient are not clear,which affects the quality control of steel ingots,and also affects the accuracy of numerical simulation methods for studying the solidification process of ingots.Therefore,there is an urgent need for a systematic study of the air gap formation process during the solidification of steel ingots and the influence of air gaps on the interfacial heat transfer coefficient.In this research,the hot casting and solidification experiments were carried out at the pouring temperature of 1550 ? and 1600 ?,Using thermocouple thermometers and high-precision dial gauges,the temperature and displacement of the mold and ingot during the solidification process were measured with time.Using the mold and ingot temperature data obtained from experiments as known conditions,the inverse calculation module of Pro CAST software was used to calculate the comprehensive interfacial heat transfer coefficient of the mold-ingot interfacial,The change law of interfacial heat transfer coefficient with time and temperature was obtained.By comparing and analyzing the experimental results,the influence of air gap on the comprehensive interfacial heat transfer coefficient is studied,and the interfacial heat transfer coefficient at different pouring temperatures is also studied.After sorting,calculating and analyzing the experimental results,the research results obtained show that:?1?After the molten steel is poured,the initial temperature of the ingot drops sharply,then the cooling rate drops significantly,and finally the temperature changes gently with time;after the mold is heated,the temperature rapidly rises,and after reaching the maximum value,it cools down at a relatively gentle rate.In general,the temperature of the narrow side is higher than that of the wide side.?2?The experimental casting uses a rectangular mold.Air gaps occur on both the narrow side and the wide side,and the air gap on the narrow side is larger than the wide side.The formation time of the narrow plane air gap is 160 s and the formation time of the wide plane air gap is 100 s.The relationship between the narrow plane and the wide plane air gap over time in 2500 s is obtained through the relationship between the air gap thickness and time: ?_k =0.00152 t +0.03921 and ?_H =0.00113 t +0.01574?3?After calculating the interfacial heat transfer coefficient,it can be obtained that the interfacial heat transfer coefficient between the Q235 ingot and the mold as a whole varies between 600 W·m^-2·?^-1 and 3200 W·m^-2·?^-1.?4?By comparing the calculated interfacial heat transfer coefficient with the formation of the air gap,the influence of the air gap on the interfacial heat transfer coefficient can be divided into four stages.In the first stage,the interfacial between the mold and the ingot is zero,the air gap at the interfacial is zero,and the value of the interfacial heat transfer coefficient is about 2700 W·m^-2·?^-1 to 3000 W·m^-2·?^-1.In the second stage of the formation of the air gap at the interfacial between the mold and the ingot,the relationship between the thickness of the air gap and the heat transfer coefficient at the interfacial is: h =-1571.13d +2850.In the third stage,the air gap at the interfacial between the mold and the ingot increases.The relationship between the air gap thickness and the interfacial heat transfer coefficient is: h =-674.67d +2134.78.In the fourth stage,the air gap thickness continued to increase but the interfacial heat transfer coefficient tended to be flat.The interfacial heat transfer coefficient remained basically at about 600 W·m^-2·?^-1.