Strain-rate dependent workability and plastic flow instability of a (Nb+V) stabilized microalloyed steel

In the current work, the strain-rate dependent workability and flow instability has been investigated in a (Nb+V) stabilized microalloyed steel. Uniaxial compression tests were conducted in intercritical and single phase austenitic temperature domain (700-1100 C) at several strain-rates (0.01-10 s(-1)), using a thermo-mechanical simulator (Gleeble (R)-3800). The results show that the flow stress increases at higher strain-rates. A good sample processing window arises at medium strain-rates (0.1-1 s(-1)); whereas, flow instability (serrations) at high (10 s(-1)) and low (0.01 s(-1)) strain-rate plastic deformations. After a detailed sample characterization, it appears that the reasons for flow instability during the hot and warm deformations are the formation of micro-cracks or void nucleation at low (0.01 s(-1)) strain-rate; whereas, flow localization and shear banding by adiabatic heating at a higher strain-rate (10 s(-1)). In both cases (0.01 and 10 s(-1)), the serration arises at periodic intervals with negative strain-rate sensitivity. At a higher strain-rate (10 s(-1)), the flow instability dominates till 1100 C, because of relieving the deformation-induced stored energy mainly by dynamic recovery. At lower deformation temperatures (i.e., 700-800 C), fine ferrite grains nucleate around shear bands by the diffusional transformation of austenite. The dynamic recrystallization is either absent or incomplete in agreement with the experimental determination of Tnr (non-recrystallization temperature) and texture analysis. The overall flow instability characteristics in terms of temperature, strain and strain-rate substantiate well with the dynamic materials model, by the superposition of flow instability and power dissipation efficiency, on revisiting the processing map.

Automated summary

The study investigated the strain-rate dependent workability and flow instability in a (Nb+V) stabilized microalloyed steel, revealing an optimal sample processing window at medium strain-rates and flow instability at high and low strain-rates. The flow instability characteristics aligned well with the dynamic materials model, showing periodic serrations with negative strain-rate sensitivity.