Numerical Modeling Of Microstructure Evolution During Multi-Pass Hot Rolling In Carbon Dual-Phase Steels

Multi-pass hot rolling is an important manufacturing process for carbon dual-phase steel.It combines plastic deformation,cooling,and solid-state phase transformation,and therefore adjusts macroscale material shape,microstructure and mechanical behavior.In this thesis,simulations are conducted to investigate the evolution of microstructure,stored deformation energy distribution,and solute distribution during different hot rolling processes in an Fe-0.07C-1.2Mn(wt.%)steel.Detailed analyses demonstrate the mutual effect of different microstructure evolution mechanisms and the influence of thermo-mechanical parameters on grain refinement,providing guides for the optimization of rolling processes.Mesoscale cellular automaton(CA)models coupled with the macroscale finite element method(FEM)are developed to simulate the recrystallization and austenite(γ)-ferrite(α)phase transformation during multi-pass hot rolling of steels.FE simulations are utilized to obtain the macroscale temperature field and strain rate during multi-pass hot-rolling processes.CA models are employed to simulate mesoscale metallurgical phenomena,including dynamic/metadynamic/static recrystallization and dynamic/staticγ-αphase transformation.The solute drag(SD)effect is coupled in theγ-αphase transformation CA model to take into account the influence of short-range manganese diffusion on theα/γinterface migration.The coupled CA-SD model is applied to the simulation of dynamicγ-αphase transformation for the first time.The austenite recrystallization during a 6-pass continuous hot-rolling process is investigated by the developed CA model.The simulation of theγ-αphase transformation during the subsequent cooling process is based on the paraequilibrium condition,coupling with the solute drag effect.Simulation results show that the average austenite grain size is reduced from～174μm to～20μm.Metadynamic recrystallization and static recrystallization dominate the microstructure evolution.The kinetics of recrystallization in the fifth pass and interval is the fastest one due to an integrating effect of parameters,such as high strain rate and moderate rolling temperature.Increasing strain rate benefits the static recrystallization in competing with the metadynamic recrystallization.The stagnant stage induced by the solute drag effect facilitates the nucleation of ferrite.The as-rolled microstructure(F_α,～0.93;(?)_α,～10μm)simulated by CA agrees well with the experimental observation(F_α,～0.93;(?)_α,～10μm).Then,the CA model is applied to the simulation of metallurgical phenomena during a6-pass dynamic strain-induced transformation(DSIT)hot rolling process,involving not only dynamic/metadynamic/static recrystallization,but also dynamic/staticγ-αphase transformation.The simulation renders a clear visualization of complicated microstructure evolution such as the interaction between recrystallization andγ-αphase transformation,and the dynamicγ-αphase transformation in a multi-pass process.Analyses focus on the contribution of the stored deformation energy to the kinetics ofγ-αphase transformation.Results show that the average grain size is reduced from～174μm to～3μm.Dynamicγ-αphase transformation is the dominant mechanism for grain refinement in the DSIT process.High strain rate and strain would promote ferrite nucleation on the austenite micro-shear band and therefore refine the grain structure.The addition of stored deformation energy elevates theγ-αphase transformation temperature and the solute drag effect is beneficial for the stability of DSIT ferrite.The simulated as-rolledα-fractions(F_α,～0.95)and averageα-grain sizes((?)_α,～3μm)subjected to the 6-pass DSIT process agree reasonably well with corresponding experimental observations(F_α,～0.94;(?)_α,～5μm).