The new generation steels have got more attentions due to the requirement of economic development. One of the important researches is enhancing the strength and the ductility using the cost-effective technology. The refinement of microstructures is one of the main approaches to enhance the strength and the ductility. For the ferrite-pearlite steels, the coupling between the deformation and the austenite-ferrite transformation is a useful method of grain refinement. Therefore, it is very important to develop a quantitative relationship between the processing of steels and their microstructure in order to produce tailor-made material by designing the composition and controlling the processing. In this dissertation, the mesoscopic models of the coupling processes between the deformation and the austenite-ferrite transformation in low carbon steels are built.
The crystal plasticity finite element model is built firstly for modeling the hot deformation of austenite at mesoscale. The simulation results show that the space distribution of the mechanical behaviors of plastic deformation is non-uniform. The orientation relationship of neighboring grains can affect this distribution intensively. The further analysis shows that this non-uniform distribution results from the interaction of the active slip systems. The other result induced by plastic deformation is the increasing austenite grain boundary areas with the increasing strain. Commonly, the increasing rate of the grain boundary areas simulated by crystal plasticity finite element model is larger than that by uniform-deformed geometry models.
For simulating the influences of austenite deformation on the subsequent austenite-ferrite transformation, a sequential integration model is built based on the crystal plasticity finite element model and the Monte Carlo model of phase transformation. The simulation result shows that the plastic deformation of austenite can obviously accelerate the kinetics of the subsequent austenite-ferrite transformation. Both the long-range diffusion of carbon diffusion and the short-range diffusion of Fe lattice at phase interfaces can be accelerated by the deformation. The simulated microstructure evolution indicates that the plastic deformation can markedly increase the ferrite nucleation density. It should attribute to three reasons: (1) the increased austenite grain boundary area due to the deformation, (2) the increased ferrite nuclei number per unit area at austenite grain boundaries, (3) the formation of high stored energy regions at austenite grain interiors induced by deformation. The spatial distribution of the ferrite grains is inhomogeneous because of the heterogeneous distribution of the stored energy.
The growth behavior of the individual ferrite and its effects on the ferrite grain sizes are investigated by coupling a Monte Carlo coarsening model. The six classes of grain growth behavior, i.e. parabolic growth, delayed nucleation and growth, temporary shrinkage, partial shrinkage, complete shrinkage and accelerated growth, can be found during the austenite decomposition. The former three growth behaviors are in accord with the observation by 3DXRD experiments while the latter three growth modes are not observed in literatures. The complex behaviors of ferrite growth result from the overlapping of carbon diffusion (soft impingement) around the ferrite grains and the coarsening of neighboring ferrite grains (hard impingement) due to the grain boundary curvature. For the traditional thermomechanical processing in low carbon steels, the coarsening of ferrite grains during the early stage of transformation might attenuate the effectiveness on grain refinement.
For investigating the dynamic microstructure evolutions (i.e. the deformation and the microstructure evolution happen simultaneously), a mesoscopic synchronous integration model is developed. The deformation induced dynamic transformation (DIDT) of a Fe-C alloy above Ae3 temperature based on this model. The influence of deformation parameters, including temperature and strain rate, on the microstructure evolution and the stress-strain curves are discussed. The simulation results show that the competition between the DRX of austenite and austenite-to-ferrite transformation causes the different microstructures and changes the shape of the stress-strain curves for the different deformation parameters. As a result of this competition, the ferrite fraction is found to oscillate during the DIDT. The stability of the induced ferrite fraction affected by the ferrite-to-austenite reverse transformation is assessed by simulating its kinetics during the isothermal holding after deformation.
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