Control Of Microstructure For High Strength Automotive Frame Steel And Development Of TMCP Processing

High strength steels for automotive frame have been widely applied because of the strict requirements which were ordered by the rapid development and drastic competition of the Motor Industry. According to the principle of reduced steel rolling technology, the works of this dissertation were carried out integrating with the resource advantages in Panzhihua Iron and Steel Company, and the evolving rule of the microstructure combining with TMCP(TMCP-Thermic Machine Control Process) for high strength microalloyed steels with vanadium or titanium for automotive beam were investigated. Utilizing thermal simulation experiments, the effects of recrystallization behavior and processing parameters on microstructure were investigated; based on which the experimental and commercial tests were preceded. In order to obtain the effects of rolling processing parameters on the mechanical properties of the products, series of tests were adopted including uniaxial tensile test, low temperature impact test and bending test. Finally, the industrial production was attained in the hot rolling production line of Panzhihua Iron and Steel Company.The main works involved as follows:1)The austenite static recrystallization behaviors of V/Ti microalloyed steels were investigated using double-hit hot compression test. The results indicate that control rolling in the non-recrystallization zone could be easily implemented in the relatively low rolling temperature with addition of vanadium and titanium.2) Precipitation behaviors of V-microalloyed steel were studied through the thermal simulation experiment and hot rolling test. The results shows that deformation induced precipitation of V(CN) occurred in austenite at the temperature below 950℃;the amount of vanadium carbonitride precipitated in ferrite at the temperature between 620-660℃was relatively more than the temperature below 600℃.3) The effect of vanadium and titanium on phase transformation in cooling stage was investigated. The results show that the ferrite transformation was accelerated with increasing of vanadium content. Both ferrite and pearlite transformations could be promoted when the vanadium content goes to a high level. The effect of titanium is similar with vanadium on the transformation.4) Effects of deformation processing parameters on the microstructure and property were discovered through hot rolling experiment. The results indicate that ferrite grains could be effectively refined by decreasing the finish rolling temperature, while both refinement of grains and transformation strengthening were obtained with lowering the coiling temperature. But the problem is that lowering the coiling temperature is unbeneficial to the precipitation strengthening of vanadium and titanium.5) Reduced rolling processing for the production of P590L was determined utilizing the commercial tests, that is appropriate controlling of finish rolling temperature and coiling temperature, and full utilization of grain refinement strengthening and precipitation strengthening in order to improve the strength and toughness. The analysis of microstructure and property shows that the low-cost P590L has possessed well combination property, cold formability and weldability.