Mesoscopic Modeling And Simulation Of Microstructural Evolution In Fe-C-Mn Steels

As the most important structural materials and the most widely used metal materials,steels with good properties,low costs and recycling have been widely used in modern industry.The final properties of steel products are closely related to their microstructures.Microstructures of steels are characterized by complexity and diversity.The microstructure morphology,grain size,solute distribution and texture of different phases in the steel have significant effects on the mechanical properties.Therefore,it is of great significance to accurately understand the microstructure behavior in the process of manufacturing,processing and use,and to illustrate the relationship between microstructure and properties for the optimization of steel materials processing technology and the improvement of product performance.This dissertation carries out modeling and simulation research on the microstructure evolution behavior during heat treatment and loading from the perspective of mesoscale material calculation.The physical mechanism of microstructure behavior of steel is deeply understood.The relationship between microstructure and mechanical properties is described,which provides theoretical support for the development of high performance advanced steel based on microstructure adjustment.Firstly,the diffusional solid phase transformation and microstructure evolution during heat treat of steel are modelled.The mesoscale cellular automaton model is established to study the microstructure evolution of austenization phase transformation and the austenite-ferrite transformation during the intercritical annealing of Fe-C-Mn ternary element steel.The effects of phase interface partition of substitutional elements on phase transformation kinetics,microstructure and concentration field evolution are studied from the perspective of thermodynamics in detail.By introducing the contribution of substitution elements into the chemical driving force,the multi-stage kinetic characteristics during austenization from ferrite/pearlite microstructure are described.The effect of substitution element on local equilibrium condition at the phase interface is studied.The austenitizing phase transformation kinetics and solute distribution characteristics at different annealing temperatures are quantitatively described.By incorporating the solute drag effect,the interaction between the partition of substitution element Mn and the phase interface during austenite-ferrite transformation is quantitatively described.Secondly,the deformation behavior of multiphase microstructure at grain scale and the deformation coordination and strain distribution between phases are calculated.Based on crystal plasticity theory,the model considering plastic slip and deformation induced phase transformation is established.The mechanical stability of single-phase austenite is studied by modeling the plastic slip and deformation induced martensite phase transformation behavior during deformation.It is found that the slip system activation during austenite deformation is obviously orientation dependent,which is not only affected by Schmid factor,but also related to the orientation dependent mechanical response of austenite when straining along different directions.Besides,the deformation induced martensite transformation also has orientation dependence.Elastically normalized Schmid factor is an effective measurement for orientation dependence of austenite mechanical stability.It is also found that newly formed martensite/austenite phase boundary inhibits dislocation movement,leading to dynamic Hall-Petch effect.The established model is used to calculate the orientation dependence of mechanical behavior of austenite with different texture,which provide a theoretical foundation for the adjustment of austenite mechanical stability by orientation design.The effect of chemical boundary on dislocation movement is further studied at the meso-scale by using the crystal plasticity model.When austenite grain contains chemical inhomogeneous,the chemical boundary in single-phase austenite will affect the local plastic strain redistribution,which is mainly reflected by the difference of stress gradient and deformation induced phase transformation behavior on both sides of the chemical boundary.Finally,mesoscale deformation behavior modeling and microstructure evolution simulation are combined to achieve the quantitative study of microstructure deformation and subsequent heat treatment process.Microstructure-based numerical modeling of the deformation heterogeneity and ferrite recrystallization in a cold-rolled dual-phase(DP)steel has been performed by using the crystal plasticity finite element method(CPFEM)coupled with a mesoscale cellular automaton(CA)model.The microstructural response of subsequent primary recrystallization with the deformation heterogeneity in two-phase microstructures is studied.The simulation results indicate that the deformation of multi-phase structures leads to highly strained shear bands formation in the soft ferrite matrix,which produces grain clusters in subsequent primary recrystallization.The early impingement of recrystallization fronts among the clustered grains causes mode conversions in the recrystallization kinetics.Reliable predictionsregarding the grain size,microstructure morphology and recrystallization kinetics can be made by comparison with the experimental results.The influence of initial strains on the recrystallization is also obtained by the simulation approach.