Developments in High Strength Steels with Duplex Microstructures of Bainite or Martensite with Retained Austenite: Progress with Quenching and Partitioning Heat Treatment

Attention has been given to the development of new higher strength steel grades by employing duplex microstructures of bainitic or martensitic ferrite with retained austenite. Such microstructures rely first upon appropriate alloying to stabilise austenite against thermal decomposition, either isothermally to bainite or athermally to martensite. The alloying, typically by involving Si additions, promotes chemical stabilisation indirectly through suppression of carbide precipitation and hence increase of austenite carbon concentration. The most recent novel concept, aimed at higher strength martensitic grades, combines enhanced Si alloying with an interrupted quench, followed by an anneal to allow carbon migration from supersaturated martensite regions to untransformed austenite, a process which has become known as quenching and partitioning, or Q&P. Recent progress in understanding the detailed evolution of microstructure during the Q&P heat treatment, and the potential for exploration of the resulting microstructures in various steel grades, will be described.