Numerical Simulation And Process Optimization Of Hot Reversible Rolling Process Of Large-Sized Special Steel Rod

As a kind of section rolling technology, large-sized rod rolling can produce non-quenched and tempered (NQT) steel, bearing steel, tube blank steel, spring steel, etc. It is characterized in high production efficiency and remarkable economic benefits, and plays an important role in the development of national economy. However, large-sized rod has the special character contrast to small and medium-sized ones. There are still many problems in its production process. In order to accurately control the rolling process to produce the large-sized rods with high properties, technique and accessional value, it is necessary to research the distributions and evolutions of macroscopic and microscopic field-variables in the rolled piece during the rolling process in detail. Due to the complexity of rolling process and the cost, the traditional on-line experimental method could not meet the need of research. In recent years, the multi-field coupled numerical simulation which is based on the physical metallurgy and finite element (FE) theories has been become an important tool for studying rolling processes. With the aid of the tool, metallurgists can realize the virtual rolling. They can not only research the exiting rolling technologies but also optimize rolling processes and predict product property. Based on the previous work, by combining FE simulation and experiment, the thesis pays attention to multi-field coupled numerical simulation of the hot reversible rolling process of large-sized special steel rod. The main research contents and conclusions of the thesis are as follows:1. In order to perform the multi-field coupled numerical simulation of the hot reversible rolling process of 38MnVS6 NQT Steel, firstly, the dynamic recrystallization (DRX) tests, metadynamic recrystallization (MDRX) tests, static recrystallization (SRX) tests, austenite grain growth tests and isothermal transformation tests of 38MnVS6 steel were carried out. The austenite grain evolution behaviors and isothermal transformation behaviors of 38MnVS6 steel were studied. Then, a whole set of mathematical models and isothermal transformation curves which were used for investigating the microstructural evolutions of the steel rod during the hot reversible rolling process were obtained.2. Based on the actual rolling production line and rolling schedule, the multi-field coupled FE models of hot reversible rolling process of large-sized special steel rod were developed by the FE software MSC.Marc and its secondary development function. Then, the thermal-mechanical-microstructural coupled phenomena and thermal-transformation coupled phenomena of large-sized 38MnVS6 steel and GCrl5 bearing steel (GCrl5 steel) rods during the hot reversible rolling process were simulated. The distributions and evolutions of temperature, strain, strain rate, austenite grain size and undercooled austenite phase transformation products were analysed detailedly. The simulation results of temperature, grain size and phase transformation products are in good agreement with measured ones. The comparisons show the validity of the FE models.3. The distribution and evolution rules of field-variables in the rolled piece during the hot reversible rolling process of different-sized 38MnVS6 and GCrl5 steel rods were analysed and compared detailedly. It is shown that the combined action of temperature, strain, strain rate, and so on, DRX, MDRX, SRX and grain growth occur, which makes the austenite grain size in the rolled piece decrease continuously. There are similar trendency of field-variables in the rolled piece during the rolling process of different-sized steel rods. But the larger the size of the rod, the higher the temperature and the more uneven the field-variables in the rolled piece during the rolling process, which leads to the increase of the grain size and its inhomogeneity with the increase of rod size. Especially to ? 150mm GCrl5 steel rod, the austenite grain size at the center is 88pm, and the difference of the austenite grain size between surface and center is 72?m. The recrystallization and grain growth behaviors of GCrl 5 steel are more sensitive to rolling parameters than those of 38MnVS6 steel.4. In order to improve the microstructure of the rolled rod, four type of rolling controlled-temperature schemes for large-sized special steel rod were proposed, and a variety of rolling technologies were selected from the schemes to optimize the rolling process of ?150mm GCrl 5 steel rod. Then, based on the optimized results of typical technologies, the optimization was obtained after comapring the merits and demerits of them. Finally, the rolling process of other-sized 38MnVS6 and GCr15 steel rods were also improved by the optimization. It is shown that the optimization can not only reduce the temperature at the center of the rolled piece during rolling process effectively, but also reduce temperature difference in the rolled piece, which can imprvoe the austenite grain size and its distribution in the rolled rod. Based on these optimization processes, a special steel company optimized its large-sized special steel rod rolling production line, which further improved the quality of rolling products.