Evolution of Microstructure and Carbon Distribution During Heat Treatments of a Dual-Phase Steel: Modeling and Atom-Probe Tomography Experiments

The temporal evolution of microstructures and carbon distributions in a Fe-0.323C-1.231Mn-0.849Si (mol pct) dual-phase steel during heat treatments are simulated using a two-dimensional cellular automaton model. The model involves austenite nucleation, phase transformations controlled by ferrite (α)/austenite (γ) interface mobility and the local carbon concentration, and long-range carbon diffusion. It is also coupled with a solute drag model to account for the effect of substitutional elements on the interface migration. The results show that after holding at 800 °C for 300 seconds the transformed γ-volume fraction is lower than the paraequilibrium prediction. During subsequent cooling at 6 °C s−1, the γ → α transformation takes place after a stagnant stage; the carbon concentrations in both the α- and γ-phases increase and become non-uniform. When cooled below 450 °C, the γ-volume fraction is nearly unchanged. A small amount of carbon enriched martensite, transformed from the remaining γ-phase, exists in the room temperature microstructure. The simulated microstructures and carbon concentrations in martensite compare reasonably well with the experimental micrographs and atom-probe tomographic measurements. During tempering at 400 °C, martensite decomposes and the carbon concentration in the α-matrix increases. The simulation results are used to understand the mechanisms of yield strength variations after different heat treatments.