Research And Application Of Heat Transfer Model And Thermal Stress Model In Solidification Process Of Billet Continuous Casting

In the continuous casting process, the internal defects which can be formed in cast material are due to inappropriate casting operation and improper secondary cooling water distribution, especially the defects of cracks represented by the midway crack and centreline crack which cover over 50% of total quality problems. In order to avoid internal cracks and other defects, it is necessary to make studies on the formation mechanism of internal cracks and control the process of solidification. The casting conditions such as pouring temperature and casting speed fluctuate frequently caused by change of equipments, process and rhythm etc, so that the heat transfer model which is built based on the static conditions cannot meet the requirements of making high-quality billet. On the other hand, the formation mechanism of internal cracks should be studied in order to avoid internal cracks, which will offer theoretical grounds for the preventing of the occurrence of internal cracks and the optimization of secondary cooling water flow to improve the strand quality. Thus, it is of practical significance to study the real-time heat transfer model and thermal stress model to improve the billet quality.With the internal crack of solidification process as the research object, and eliminating the internal crack as the final target, this dissertation was formed. In order to avoid the internal crack, the reason of internal crack generation was studied. The stress model was established to analyse the stress distribution and influence factors. The heat transfer model was systemically studied because it is the basis of the thermal stress model. The heat transfer model and thermal stress model were applied to the industrial caster. The main content and innovations are as follows:(1) Establishing two-dimensional thermal elasto-plastic stress model and researching the formation mechanism of internal cracksThe internal cracks such as midway crack and centreline crack are the main internal defect. A two-dimensional thermal elasto-plastic stress model was established to analyze the formation mechanism of internal cracks.The billet thermal stress distribution and the formation mechanism of internal cracks were analyzed by the model. It was presented that the internal cracks were caused by excessive surface reheating and solidification velocity disproportionation. The cracks occur between shrinkage boundary and solidoid boundary and spread to the solidoid boundary. Meanwhile, the high superheat and frequent casting speed fluctuation aggravated the expansion of cracks. By means of analysis of mechanical properties of steel and model, the surface temperature fluctuation should be reduced and the reheating should be decresased in each section of secondary cooling zones.(2) Establishing the real-time heat transfer and solidification model and analysing the dynamic performance of billet temperature fieldSecondary cooling control is one of the key techniques of influencing the quality of billet. The solidification process in the secondary cooling should be controlled, which is equal to control the temperature field of billet.Owing to the influence of high-temperature, water vapor, water film and scales on the strand surface, etc in the secondary cooling zone, it is difficult to measure the billet surface temperature accurately, let alone the internal temperature. According to the frequent changes of casting processes in the continuous casting production, a two-dimensional real-time heat transfer and solidification model was established.In order to correct and test the real-time heat transfer and solidification model, the surface temperature and shell thicknesses were measured by the thermal imager and nail shooting respectively. The the influence of the grid division to the reliability of the model was analyzed. The dynamic performance and response to operation conditions were tested by changing casting conditions. The maximum deviation between the calculated surface temperature and measued ones were below 10°C at measured point by changing operation conditions, the foundation which the real-time model was on-line application and the thermal stress model was established.(3) Application research of thermal stress model and real-time heat transfer model for the control system of billet continuous castingThe application research of steel grade Q235 and HRB400 were performed in a steel plant. On the basis of model calculation and the results of experimentation, the improper secondary cooling water distribution which leads to high reheating between sections in secondary cooling zone was the major cause of internal cracks. Meanwhile, the high superheat and frequent casting speed fluctuation aggravated the expansion of cracks. The secondary cooling water was optimized, and the stress distribution was calculated before and after the optimization of secondary cooling water. The water distribution coefficients of casting speed and superheat were obtained. The fluctuation of surface temperature was reduced greatly during casting speed abrupt changing beacuse the superheat and effective casting speed were imported in the dynamic secondary cooling control systemAfter the system was applied to the field, the analysis of macrostructure random specimens of six months showed that the Grade Q235 centreline cracks were basically eliminated and the midway cracks above 1.0 degree has been dropped from 10.3%o 4.7%. The equiaxed grains diameter has risen up from 28～32mm to 35～42mm, and other defect grades have been reduced; the Grade HRB400 centreline cracks were basically eliminated and the midway cracks above 1.0 degree has dropped from 12.7%to 7.4%. The quality of products has been improved obviously...