Research On Dynamic Recrystallization Behavior And Microstructure Simulation Of Q550D Ultra-Lowcarbon Bainitic Steel

The ultra-tow carbon bainitic steel still has high strength and excellent lowtemperature weldability when it has good toughness at the same time. Its excellentperformance depends on the microstructure after thermal deformation. Therefore, thethermal deformation behavior and microstructure evolution of the ultra-low carbonbainitic steel have been investigated, the results from this project are useful for theimprovement in its comprehensive properties and production process. The dynamicrecrystallization behavior and prediction model of Q550D ultra-tow carbon bainiticsteel have been first studied by using isothermal constant strain rate compression testsand metallographic analysis system. Further, the simulation on the microstructureevolution during thermal deformation has been carried out by using the microstructuremodule of DEFORM-3D. The main research work includes:Firstly, the flow properties of Q550D ultra-tow carbon bainitic steel were studiedby isothermal compression of cylindrical specimens at1000~1150℃and strain rateof0.01~0.1s-1with Gleeble-3500thermo-simulation machine. Dynamicrecrystallization law was studied by analyzing the stress-strain curves. Thedeformation temperature of the steel prone to dynamic recrystallization wasdetermined, and the temperature range is from1100℃to1150℃in the currentdeformation condition. Furthermore, the critical conditions for DRX and peakstress/strain were determined by the relationship between work hardening rate andflow strain. The effect of deformation temperature on dynamic recrystallizationstarting time was analyzed by the maps (RRT diagram) of the relationships betweenthe dynamic recrystallization starting time and the temperature as well.Secondly, according to the theory of recrystallization grain nucleation andgrowth as well as the basic characteristics of recrystallization kinetics, themathematical model of the DRX kinetics, the dynamic recrystallization grain sizemodel, p-lnz relationship model, p-lnz relationship model and c-lnzrelationship model for low-carbon Q550D steel were developed respectively. Thedeformation activation energy of386.43kJ/mol was evaluated by linear regressionanalysis. And the flow stress constitutive equation of Q550D ultra-tow carbon bainiticsteel during hot compression was obtained. Finally，The dynamic recrystallization models of Q550D ultra-tow carbonbainitic steel were established during thermal deformation on the platform ofDEFORM-3D, and the microstructure evolution of Q550D ultra-tow carbon bainiticsteel was simulated under different deformation conditions by using the cellularautomaton (CA). The simulation results show the microstructure of Q550D ultra-towcarbon bainitic steel was greatly improved during thermal deformation and thedynamic recrystallization happened as well as grains became uniform refinementduring the simulation which was consistent with experimental results. Besides, theaverage grain size and the dynamic recrystallization fraction obtained by simulationare agreeable well with the results of the experiments, that was, relative error of theaverage grain size was less than8.6％and the dynamic recrystallization fraction was100％.