New insight into the role of austenite grain size in the bainitic transformation of low carbon steel

Bainitic transformation plays a crucial role in determining the mechanical properties of low carbon steel, especially for applications in the automotive industry. Regarding the effect of prior austenite grain size (PAGS), extensive studies have focused on the acceleration or deceleration of bainitic transformation kinetics but have relatively neglected the formation of hierarchical microstructures. Through a conjoint analysis of transformation kinetics and variant grouping, a new insight into the role of PAGS in the bainitic transformation behavior of carbide-free low carbon steel is proposed. A systematic study involving three distribution states of PAGS obtained in different austenitizing temperatures reveals significant differences in bainitic transformation kinetics and heterogeneous microstructures, but little influence on final mechanical properties. Different from the traditional consideration of thermodynamic and kinetic constraints on bainitic transformation, the strain accommodation arising from crystallographic variant grouping, as another degree of freedom, can greatly alter the effect of PAGS. Through the separate fitting of the kinetic model for different PAGSs, larger grains exhibit lower activation energy for autocatalytic nucleation, Q A , facilitating the activation of multiple variant grouping to accommodate strain accumulations, as compared to smaller grains. Although the kinetic model assumes constant strain energy, according to the thermo-kinetic correlation, the role of variant accommodation is mainly reflected in the change of Q A , whose decrease corresponds to the strain energy release and multiple variant formation. Finally, the similar evolution of driving forces and generalized stability reveals a negligible difference in the mechanical properties.