Simulation Of Microstructure Evolution During Intercritical Annealing Of Cold-rolled Dual-phase Steel

Dual-phase(DP)steel,as the flagship of the first generation advanced high-strength steels,has been widely used in the automotive industry due to the combined properties of high strength and good formability.Continuous annealing is widely used in the steel industry to produce DP steels,during which a series of complex metallurgical phenomena will happen,such as ferrite recrystallization,cementite spheroidization,austenite formation and austenite-to-ferrite transformation.These phenomena may proceed concurrently or consecutively with complex local interactions between each other,which makes the resulting microstructure closely process-related.Consequently,it is quite essential to develop quantitative models to build connections between the processing route and the microstructure evolution in order to achieve the desired material properties through design optimization of material compositions and the processing parameters.In this work,a combined methodology of mesoscopic modeling is developed to investigate the microstructure evolution during continuous annealing of DP steels.A digital material representation(DMR)algorithm is developed to generate the initial microstructural geometry of dual-phase materials based on real microstructural data,which is then incorporated into the CPFEM model to simulate the microstructure deformation during virtual tensile test.The DMR algorithm takes the advantages of the image processing and CA normal grain growth model,which can then be used to automatically generate the microstructural geometry from metallographic images and EBSD data.Thus,the microstructural model as generated by the DMR algorithm can provide reliable initial model inputs for the subsequent CPFEM and CA simulations by reflecting the nature of the microstructural topology between different phases and grains as well as the grain orientations in real materials.CPFEM simulation results reveal the heterogeneous microstructural deformation behavior of the dual-phase polycrystalline structures under virtual tension.Strong strain and stress partitioning between different phases and grains are found within the deformed the microstructure,forming sharp deformation bands across the microstructure during deformation.The simulated distribution of the deformation bands and macroscopic stress-strain response are in good accordance with the experimental results,which confirms the reliability and accuracy of the CPFEM modeling approach used in this study.Cellular automata modeling and simulation of ferrite recrystallization during annealing of a cold-rolled DP steel is then carried out by coupling with CPFEM simulations of microstructure deformation in order to take the microstructure deformation heterogeneity into account.Microstructure topology,grain orientation and the deformation energy within the deformed microstructure as calculated by CPFEM simulations are mapped to the CA lattice as initial states for recrystallization simulation.Simulations results show that the inhomogeneous distribution of deformation energy in the deformed microstructure results in non-uniform nucleation and growth behavior during ferrite recrystallization,which produces an inhomogeneous microstructure with non-uniform grain size distribution after full recrystallization.The simulated recrystallization kinetics and morphology are in good accordance with the experimental results.Austenite forms concurrently with the dissolution of ferrite and cementite during intercritical annealing of pearlite structures.Detailed simulations on the scale of individual cementite plate are then carried out to study the interface evolution during austenite formation within the lamellar pearlitic structure.The moving austenite interface driven by the Gibbs-Thomson effect is found to change from an initial transient state to a final steady state with constant migration rate and interface shape during austenite formation from an ideal single-layered lamellar pearlite.Wave-shaped moving austenite interface is gradually formed due to the carbon concentration gradient along the migrating front during pearlite dissolution.However,when taking the structure of pearlite colonies into account,steady-state migration mode of the moving austenite interface can never be reached due to the geometric uncertainties of the pearlite colonies.Inhomogeneous distribution of carbon concentration within neighboring ferrite and austenite is formed during cementite dissolution and spheroidization driven by the Gibbs-Thomson effect.Through-process simulation of the microstructure evolution during intercritical annealing of a cold-rolled DP steel is finally investigated by a coupled CPFEM-CA approach,including ferrite recrystallization,austenite formation and austenite-to-ferrite transformation.A cellular automaton model with dual-scale cells is developed to describe the constitutional phases of ferrite and cementite within the pearlite structure so that carbon redistribution to the neighboring ferrite and austenite during cementite dissolution can be considered.The simulation results dynamically revealed the microstructure evolution kinetics and morphology under the interaction of ferrite recrystallization and austenite formation during annealing.A two-stage transformation kinetics is presented during austenite formation,which corresponds to fast pearlite-to-austenite and slow ferrite-to-austenite transformation,respectively.Ferrite-ferrite boundary nucleated austenite nuclei are found to grow either to necklace-type distributed grain boundary austenite or dispersed intragranular austenite depending on the local interactions between ferrite recrystallization and austenite formation,which is strongly affected by the heating rates and annealing temperatures.Minor changes in these interactions can cause profound changes in the transformation kinetics and the resulting microstructure.The coupled CPFEM-CA modeling approach gives good predictions of the microstructure evolution for DP steel production under a simulated industrial intercritical annealing cycle.Simulation results in terms of transformation kinetics and microstructure morphology are found to be in good agreement with experimental observations.