The influence of Nb on transformation behavior and mechanical properties of TRIP-assisted bainitic-ferritic sheet steels

In this study the concept of microalloying was applied to TRIP-aided bainitic-ferritic (TBF) sheet steel. A newly designed chemical composition was introduced including significantly reduced Si content. The impact of Nb on phase transformation, microstructure and mechanical properties was investigated. Nb addition was found to indirectly accelerate transformation kinetics through grain refinement resulting in pro-eutectoid ferrite and upper bainite formation. A change in morphology of the bainitic matrix from lath-like to globular was observed as a result of grain refinement. The amount of cementite was reduced and the amount of retained austenite increased through the addition of Nb. The changes in phase transformation and microstructure resulted in significantly improved mechanical properties comprising both elevated stretch formability and edge crack resistance.