High strength steels with exceptional combinations of strength and ductility have been obtained through a rapid thermal cycling process, designated as Flash® annealing. The exceptional properties are obtained through the formation of a heterogeneous microstructure consisting of an inhomogeneous distribution of bainite, martensite and retained austenite after Flash® annealing. The distribution of these constituents in the processed microstructure is determined by the solute concentration gradients that develop due to carbide dissolution, and austenite nucleation and growth during rapid thermal cycling. We present high-resolution, three-dimensional, phase field simulations that quantify the effect of the initial microstructure and processing conditions on the development of solute concentration gradients. Research sponsored by the Advanced Manufacturing Office of the U.S. Department of Energy under the High-Performance Computing for Energy Innovation (HPC4EI) program and performed at the Oak Ridge National Laboratory managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 for the U.S. Department of Energy.