Metastable austenite driven work-hardening behaviour in a TRIP-assisted dual phase steel

Abstract

The mechanically-induced transformation behaviour of the metastable austenite phase in a high-strength industrial TRIP-assisted Dual Phase steel was monitored in situ using high-energy synchrotron diffraction under uniaxial loading. This allowed direct quantification of the impact of the transformation of the metastable austenite phase (16 vol %), embedded in a ferrite-bainite-martensite matrix, on the work hardening behaviour of this steel. Our results show that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding. We provide experimental evidence which demonstrates for the first time that the austenite transformation increases the work-hardening contribution, σw thereby supporting a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding and continues to offset the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Further we show conclusively that martensite yielding does not occur until the completion of the mechanically induced transformation of austenite. Plastic deformation of martensite is immediately followed by local plastic instability leading to necking and ultimate failure of this material

Work hardening behaviour in banded dual phase steel structures with improved formability

“…In order to determine accurately the yield stress of these steels, it is necessary to find the transition from the pre-yield, reversible glide regime into irreversible plastic deformation. This pre-yield behaviour is best illustrated using an extended Kocks-Mecking (eK-M) plot of the workhardening rate as a function of the nominal equivalent stress [11,34]. representation of the work-hardening rate, Θ = dσf/dε vs. the flow stress σf, [35][36][37].…”

“…Recently we demonstrated how the mechanically induced austenite transformation contributes to the workhardening in non-banded DH steels. We proposed a physically-based model based on an extended Kocks-Mecking analysis, coupled with in situ high-energy X-ray diffraction data collected upon straining, to describe the effect of the mechanically-induced austenite transformation on the work-hardening behaviour in a non-banded DH steel [11]. In this work, we assessed the effect of banding on the work-hardening behaviour in DH steels, by applying an equivalent physically-based model analysis of the work-hardening behaviour on banded and 5 non-banded microstructures of the same DH steel grade.…”

Deformation-Induced Microstructural Banding in TRIP Steels

“…In the current study, aside from the initial microstructure, the microstructural observations start a few percent before the UTS. [36][37][38] No differential behavior was observed in different individual islands of hard phases and if austenite was present, it was not distinguishable in terms of deformation behavior. Nevertheless, while a detailed characterization of microstructure constituents is not given here, it would be a useful separate study in itself, like that performed in related material.…”

Effect of retained austenite stability on cyclic deformation behavior of low‐alloy transformation‐induced plasticity steels

“…The strain‐hardening exponent values are higher in the case of the HT3x ( n = 0.262‐0.274) compared with the HT2x materials ( n = 0.198‐0.205). The strain‐hardening exponent increases with increasing % RA transformation because of the premature formation of the hard martensitic phase …”

“…The strain-hardening exponent increases with increasing % RA transformation because of the premature formation of the hard martensitic phase. 38…”