Genesis of Plasticity-Induced Serrated Metal Flow in Medium-Mn Steel

An excellent strength-ductility combination offered by martensite-austenite microstructure in medium-Mn steels draws special attention for automotive applications. However, yielding of the steel is often compromised in the form of serration, with a jerky metal flow and deformation banding, by the reason yet to be settled. Thus as per the practice, a hot-rolled sheet after intercritical annealing at 650 °C is subjected to the current investigation. The intercritical annealing reveals an undissolved-unoccupied state attained by carbon, which readily interacts with the sample dislocations introduced in the microstructure during the prior processing. The interaction forms Cottrell atmospheres at annealing temperature rather than at room temperature as enumerated in metal plasticity theory, and hence, finds the clue for hitherto unexplored mechanism of dynamic strain aging. Accordingly, when fresh dislocations intercept barriers laid by the carbon-dislocation aggregates into clusters in advance, the interaction triggers serration for the proposed mechanism.