Critical strain for dynamic recrystallization during the constant strain rate hot deformation of niobium steels

The presence of peak points in constant strain rate flow curves is generally considered to indicate the onset of dynamic recrystallization (DRX). However, not all materials display clear peaks when tested under these conditions. It has been suggested that DRX is initiated when the strain reaches a critical value epsilonc that is related to the peak strain epsilonp and that this critical strain can be detected from the inflection point in plots of the strain hardening rate theta against stress sigma, regardless of the presence or absence of peaks. This is of importance because many works regarding DRX have been carried out by physical simulation using laboratory torsion tests.;However, one of the main problems in laboratory simulation is that the inflection point for DRX is not easily determined, particularly in microalloyed steels. This problem is rendered more difficult in torsion testing because of the strain gradient along the radius of the specimen. As an alternative, compression testing can provide more accurate data regarding the initiation of DRX.;In this work, the DRX behavior of three Nb microalloyed steels was investigated using compression testing. Continuous deformation by axisymmetric compression was employed in the temperature range 850 to 1050°C at strain rates of 0.01, 0.1 and 1/sec. Stress-strain curves were obtained and then the critical strains for initiation were calculated using a method based on the presence of an inflection point at the moment of initiation. The critical strain was observed to increase when the deformation temperature was decreased or when the strain rate was increased. No significant effect of the level of microalloying addition on the DRX behavior was detected within the range studied.