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A series of plane strain compression tests were carried out in order to simulate the thermomechanical controlled processing of a 0.09wt% Nb low carbon steel, in a scheme of multipass finish rolling at 950 °C with interpass times of 10 s. It was observed that after the first two finishing passes a remarkable grain refinement can be achieved, since the recrystallisation was fully suppressed and abundant ultrafine ferrite was transformed dynamically during the deformation. The addition of a third finishing pass however, led to partial recrystallisation. A deep characterisation of the dynamic ferrite was carried out by diverse methods conducting to relevant findings that contribute to a better elucidation of the dynamic transformation. The results obtained indicated that the dynamic formation of a colony of Widmanstätten ferrite plates during deformation, initiates with the formation of a pair of self-accommodating plates followed by face-to-face sympathetic nucleation of new plates at one of the faces of the pairs of plates already formed. Furthermore, the crystal orientation within the dynamic ferrite phase was analysed with EBSD, it was observed that during the coalescence of plates, prior to the full polygonisation of grains, the ferrite adopts a transitory morphology which possesses particular crystallographic characteristics. [ABSTRACT FROM AUTHOR]

A series of plane strain compression tests were carried out in order to simulate the thermomechanical controlled processing of a 0.09wt% Nb low carbon steel, in a scheme of multipass finish rolling at 950 °C with interpass times of 10 s. It was observed that after the first two finishing passes a remarkable grain refinement can be achieved, since the recrystallisation was fully suppressed and abundant ultrafine ferrite was transformed dynamically during the deformation. The addition of a third finishing pass however, led to partial recrystallisation. A deep characterisation of the dynamic ferrite was carried out by diverse methods conducting to relevant findings that contribute to a better elucidation of the dynamic transformation. The results obtained indicated that the dynamic formation of a colony of Widmanstätten ferrite plates during deformation, initiates with the formation of a pair of self-accommodating plates followed by face-to-face sympathetic nucleation of new plates at one of the faces of the pairs of plates already formed. Furthermore, the crystal orientation within the dynamic ferrite phase was analysed with EBSD, it was observed that during the coalescence of plates, prior to the full polygonisation of grains, the ferrite adopts a transitory morphology which possesses particular crystallographic characteristics.

Abstract Nowadays, a new concept of process utilizing dynamic ferrite transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1 μm, has been proposed. This paper reports transformation mode of dynamic ferrite transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of in-situ neutron diffraction analysis during thermomechanical processing. Dynamic ferrite transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed ferrite is the main mechanism for the formation of ultrafine-grained structure. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The hot–rolled microstructures of medium Mn lightweight steels led to the formation of Mn–segregation bands due to a relatively high Mn content of 9–12 wt%, ultimately deteriorating their overall mechanical properties. In this study, a multi–pass warm rolling of 95% reduction was proposed to minimize the Mn–segregation bands in the 10 wt% Mn lightweight steel. This was followed by the detailed microstructural examinations and ex–situ tensile testing to evaluate the mechanical behavior response and deformation mechanism operating upon straining. Severe warm rolling accelerated the dynamic ferrite transformation and recrystallization in the Mn–rich austenitic bands, thus improving the microstructural homogeneity. The austenite stability was further improved so as to enable continuous strain hardening by transformation/twinning–induced plasticity (TRIP/TWIP) effects. This, thereby, led to obtain high tensile strength of 985–1201 ± 15 MPa together with large elongation of 34.3–76.0 ± 1.1%. In addition, the strain compatibility between ferrite and austenite also proves to give rise to an alleviation of strain localization at the ferrite/austenite interface, which also postponed the plastic instability and contributed to the large ductility by the coordinated TRIP/TWIP effects. • A multi–pass warm rolling of 95% reduction was proposed to minimize the Mn–segregation bands in the Mn-based lightweight steel. • Severe warm rolling accelerated the dynamic transformation/recrystallization in the Mn–rich austenitic bands and improved the microstructural homogeneity. • The austenite stability was further improved due to dynamic transformation so as to enable continuous strain hardening by TRIP/TWIP effect. • The synergic TRIP/TWIP effect and improved strain compatibility between ferrite and austenite results into an extremely high PSE value of over 74 GPa·%. [ABSTRACT FROM AUTHOR]