Inhomogenous deformation and strain-rate effects in high-strength TRIP steels

Abstract

The tensile true stress-strain behaviour of a high-strength TRIP steel was examined at temperatures of 25,66 and 150°C and for imposed nominal strain rates of 0.005, 0.05 and 0.5 min− 1. True stress, true strain and true strain-rate were determined during Lüdersband formation, uniform elongation and final necking. At room temperature where austenite stability is low, the true-stress-strain curve has a sigmoidal shape, consistent with the sigmoidal relation for the volume fraction of martensite versus strain reported for low strength metastable austenitic steels. Lüdersband formation at yielding was found to be identical with the immediate necking that occurs at higher temperatures and is a result of the low initial strain-hardening rate associated with the sigmoidal transformation behaviour. Propagation of the Lüders band and subsequent uniform flow require a high rate of strain-induced transformation at higher strains. At a nominal strain rate of 0.5 min− 1, adiabatic heating sufficiently lowered the transformation rate so that no stable uniform plastic flow occurred. The adverse effect of adiabatic heating in these steels is aggravated by the increased local strain rate associated with Lüders-band formation. If high-strength TRIP steels are to be made suitable for dynamic applications, the apparent strain-rate sensitivity must be significantly decreased by either eliminating the initial inhomogeneous flow or reducing the temperature sensitivity of the strain-induced martensitic transformation.