Thermo-Mechanical Processing of Dual-Phase Steels and Its Effects on the Work Hardening Behaviour

Abstract

This thesis focuses on understanding the relationship between the microstructure and the different work hardening mechanisms of DP steels. Through the application of various thermo-mechanical processing schedules prior to inter-critical (IC) annealing, five distinctly different microstructural variants were produced. The work hardening behaviour of the five microstructural variants was examined in terms of the true work hardening rate, θ, the instantaneous work hardening exponent, n, and the dislocation annihilation factor, h. Additionally, back stresses were measured in selected microstructural variants having similar martensite volume fraction of ~15%, using a custom-made in-plane forward-reverse shear testing fixture.

At small strains (<2%), the work hardening behaviour was found to be dominated by the introduction of back stresses and the generation of GNDs in the ferrite matrix. The work hardening response at this stage was characterized by θεp=0.5% and a minimum value in the instantaneous work hardening exponent, nmin. Both of these parameters were determined to be functions of (f/d)^1/2 (f is the volume fraction and d is the size of martensite particles), the mean ferrite grain size as well as the morphology and spatial distribution of martensite particles.

At higher strains (2-3%), a maximum value in the instantaneous work hardening exponent is reached (nmax). This parameter, which can be considered as the work hardening capacity of the material, was found to be a function of mean ferrite grain size but is independent of (f/d)^1/2. The relative contribution of back stresses was also found to reach a constant value at a similar von Mises equivalent strain. This observation suggests that at strains above those associated with nmax, other work hardening mechanisms become more important.

At strains over 4%, dislocation annihilation by dynamic recovery becomes the controlling factor for the rate of work hardening. This phenomenon is described by the dislocation annihilation factor, h, and is a function of (f/d)^1/2, the mean ferrite grain size as well as the morphology and spatial distribution of martensite particles.

Finally, it was concluded that the ideal DP microstructure will contain a uniform distribution of fine, equiaxed martensite particles in a fine, equiaxed ferrite matrix.