Plastic yielding and work hardening behavior in Q&P steel with multiphase structure

Yielding in a 0.44 wt%C Q&P steel with a multiphase structure is attributed to initiation of plastic deformation in retained austenite (RA) and carbide-free bainitic ferrite (BF). The yield stress (YS) is controlled by the strength of the RA. High strength of RA is attributed to superposition of carbon enrichment and high dislocation density. A high work-hardening rate at the 0.2 % off-set strain corresponds to a high volume fraction of RA in which the passage of dislocation multiplication from one lamella to the other crystallites may occur. Continuous yielding takes place due to increasing volume fraction of structural constituents being perfectly plastic with increasing strain from the offset one to strain at which the work-hardening rate attains the E/50 limit. The last strain ranges from 5.1 to 8.7 % that is an evidence for extended elastic-plastic transition in the Q&P steel with multiphase structure.