Microstructure and modeling of bainite transformation in deformed austenite

Two distinct types of bainite microstructures, acicular ferrite (AF) and conventional bainite (CB), have been observed in low-carbon plate steels. The primary goal of this research is to understand the basic thermodynamic and kinetic mechanisms that produce the ultra-fine AF microstructure, which has a promising combination of high strength and toughness. The study consists of two parts: (1) experimental work that studied the effects of TMP variables and alloying on bainite transformation in deformed austenite with focus on the development of AF microstructure. (2) phase field modeling to simulate the bainite microstructural evolution in deformed austenite.; The bainite-start temperature (BS) is determined by the onset of the CB transformation. Increasing deformation strain (epsilonD) and decreasing austenite grain size (Dgamma) increase BS, and accelerate the overall bainite transformation. AF nucleates at deformation-induced intragranular high-angle boundaries. Only the boundaries which have a misorientation angle larger than a critical angle can act as potential nucleation sites for bainitic ferrite. AF grows as groups of multi-variant parallel laths, which can minimize the strain energy accompanying transformation. The combination of a high-density of intragranular nucleation sites, the impingement effect between different AF groups, the constraint effect of dislocation cells on AF growth and the formation of multi-variant AF laths groups results in the ultra-fine AF microstructure.; There exists a cooling rate range for predominantly AF formation and it varies with the chemical composition of the steel. Increasing Mn and Cr contents of X80 steel significantly decreases BS, and displaces the AF transformation range during continuous cooling to lower temperatures and lower cooling rates.; 2D phase field simulation shows that the bainitic ferrite first nucleates and grows from higher energy dislocation boundaries. This transformation is followed by nucleation and growth of new bainitic ferrite laths on the existing lath. The model qualitatively predicts the observed morphology of bainitic ferrite laths, although the overall microstructure does not resemble experimentally observed microstructures due to the simplifying assumptions.