Predicting Fully Bainitic Structures in Steels Under Continuous Cooling Conditions

Automotive power transfer applications require bar steels with high strength and toughness to reduce weight and prevent fatigue failure under cyclical loading. Recently, there has been increasing interest in bar steels that form as rolled bainitic structures. Bainite may offer improved strength and toughness compared to quench and tempered (Q&T) and ferrite/pearlite steels. Additionally, bainitic steels can eliminate the heat treatment step required for Q&T steels. However, developing bainitic bar steels is challenging, as no agreed-upon metric defines how easily an alloy forms fully bainitic microstructures. This work introduces quantitative definition for the ease with which an alloy forms a fully bainitic structure, which considers both the width and position of the bainite shelf. Continuous cooling transformation (CCT) data from the literature were used to create empirical relationships between alloy composition, grain size, and bainite formation. The bainite shelf widths and positions predicted by the empirical relationships were compared to measured CCT behavior to validate the models, as well as to predictions made by Thermo-Calc®. The quantitative effects of alloying elements on bainite formation, as shown by the models, are also discussed, along with their potential role in the engineering of bainitic bar steels.