Modelo para previsão da condição térmica de panelas de aciaria

Abstract

Temperature control during the whole steelmaking process has a greatimportance to achieve the quality and productivity requirements demanded nowadays.At Gerdau Açominas steelplant, the temperature control of liquid steel from tapping up to the solidification in the caster has been widely studied. Due to the long casting times, a lack of control of the steel temperature, frequently, causes interruptions in the casting process, leading to productivity and yield losses.In the present work, a model to predict the thermal condition of the steel ladles was proposed, in order to reduce the temperature variations during the steelmakingprocess.The model is based on differential heat transfer equations in a one-dimensional transient system and a explicit finite difference approach was chosen to resolve the equations. The mathematical model was calibrated adjusting the simulated results with the real data from in plant measurements at Gerdau Açominas. It was evidenced that during the heating process of a new ladle or ladle out of cycle, the temperature of the refractories close to the hot face increases quickly during the first 6 hours of heating. When the heating time is 24 hours, the temperature drop of the steel, after tapping, was reduced in approximately 80 °C, when compared with the temperature drop for a heating time of only 6 hours.The refractory on the sidewall reached a steady thermal state after 3operational cycles, while the refractory of the bottom of the ladle, due to the bigger thickness, continued to reach higher temperature values even after 4 operation cycles.The refractory with half of the original thickness achieve the steady thermal state after 2 operational cycles; however the temperatures reached were lower than those for the original hickness.For long cycle times, the intermediate heating time is important to reduce thetemperature drop of the liquid steel after tapping. However, it can be said that, with a good management of the ladles in operation, in order to reduce the times without steel,it is possible to eliminate the necessity of the intermediate heating.The more affected region by the waiting time for tapping was located between the hot face and approximately 35 mm of this. When the waiting time for tapping was more than 15 min, practically all the energy stored during the intermediate heating was lost.During the heating process, the temperature of the ladle shell increased more quickly and reached a value 45 °C higher, when the ladle was without the insulating material. It was also evidenced that the use of insulating materials increased the amount of energy stored in the refractory.As the empty ladle time increased, the refractory temperature decreased in all its layers, reaching a maximum value of 388 °C when the cycle time was equal to 500 min without intermediate heating. Up to 120 min of empty ladle, was not noticed a significant variation in the shell temperature, kept around 384 °C. The temperature drop of the liquid steel was of approximately 26 °C more than the temperature drop of the short cycle of 60 min. When the cycle reached 200 min, the temperature drop of the steel passed to 72 °C more if compared with the short cycle ladle