Effect Of Temperature Holding Hood On Temperature Difference Between Head And Tail Of Billet During Direct Rolling Process

As an important technology of saving energy,reducing consumption and reducing production cost,direct rolling technology has been adopted and implemented by more and more steel enterprises under the situation of energy exhaustion,environmental deterioration and overproduction of iron and steel industry in recent years.Although the direct rolling technology has many advantages,but the temperature difference between head and tail of billet exists when billet is transported to the rolling mill because of the different natural cooling time.When the temperature difference between head and tail of billet is large,the microstructure and mechanical property uniformity of the product will be adversely affected.In this paper,the 150 mm ×150 mm× 6 m billet is used as a research object.According to the width and rotational speed of the conveyor roller,the structural plate size and the thickness of thermal insulation material are determined,and the temperature distribution of the high temperature section of the temperature holding hood is analyzed to ensure the reasonable and reliable size of the heat preservation cover.It can also improve the service life of the heat shield.Based on the theory of heat transfer,a mathematical model of non-steady state heat transfer of three dimensional square was established to show the influence of the heat insulation cover process of the direct rolling process on temperature difference of head and tail of billet and then to compare with conventional rolling and soaking pit technology.The conclusions obtained are as follows:(1)Based on factory conditions,the height of insulation cover structural plate was 290 mm,the width was 450 mm and the thickness was 4 mm,and the modal analysis of the structural plate of this size was carried out.The minimum fixed frequency was 173.9 Hz,which is larger than the frame vibration frequency,and meets the requirements.(2)Combined with existing fractal theory of fiber material and the empirical formula of contact thermal resistance,the heat transfer process of temperature holding hood was analyzed and the temperature distribution of high temperature section was obtained and the position of highest and lowest temperature characteristic point was determined.The thickness of the insulation material was determined with the thickness of 40 mm on account of covering safety and the economic principle.(3)Temperature holding hood was made according to the above size.The temperature field analysis was carried out by the method of numerical simulation in the process of conveying head of the billet through the hydraulic shear to the rolling mill.The temperature difference between the head and tail of the cast billet was reduced from 49.8 to 28.6 degrees centigrade compared to the exposed roller table with the cover of 6 m heat preservation cover.The temperature difference between head and tail reduced by 21.2 degrees centigrade.The temperature of the numerical simulation was verified on factory.The temperature difference between head and the tail was 27.4 degrees centigrade with the error of 4.4%,which is in good agreement with the simulated value.(4)The temperature field of the billet at different time of heat sharing in the heating furnace was analyzed.According to contrast of the time,1200 s was better to carry the billet to the rough rolling mill,with temperature difference between head and tail less than 10?.Through the analysis and comparison of the process cost of the heat preservation cover and the heating furnace,the process of heat insulation cover was far lower than that of the heating furnace.It is a good choice for the common bar rolling.