Processing of harmonic type DP microstructures in the given steel and investigating deformation behavior through tensile testing and also by modelling-simulation.

Abstract

Dual phase steels (DP steels) consist of martensite phase (hard phase to impart high strength) and ferrite phase (soft phase to provide elongation). These steels have been widely used in automobile units because of their good mechanical properties providing good strength-ductility combination. Now-a-days, research has focused on development of DP steels with lean composition. Parallel to this, new processing techniques have also been developed to control the microstructure evolution which further improves the strength-ductility combination. Controlled annealing is one of the techniques used to alter the ferrite-martensite morphology of DP steels with lean composition. The present dissertation work was an attempt to extend the concept of harmonic structure design on the microstructure of DP steels and to investigate its effect on the strength-ductility combination of the steels. In the present work, harmonic structures were developed in DP steels having soft-shell of ferrite phase (three-dimensional network structure) surrounding the hard-core martensite phase. Five different annealing cycles were used to get distinct DP microstructures having different martensite distribution and volume fraction. These five different cycles belonged to two broad annealing categories (i) Continuous Annealing Line (CAL process) and (ii) Core and Shell (CAS process). The tensile properties of steels with these different DP microstructures were evaluated using tensile testing. Experimental results showed that strength of steels increased with increase in martensite volume fraction in the DP microstructure. Also, for the dual phase steels with harmonic structure, steels containing higher martensite fraction and typical core and shell type microstructure showed very limited microcracking of martensite in the tensile necking zone. Further, ferrite-martensite interface and martensite inter-lath damage activity was restricted and both ferrite and martensite revealed plastic strain compatibility. In the second major part of this dissertation work, microstructures obtained by CAL and CAS (only one out of the four microstructures processed by CAS) routes were successfully modelled using Dream.3D software. Further, simulation results using Abaqus software were able to appropriately predict the deformation behavior of DP steels. Good agreement between predicted simulation results and experimental results was observed with regards to deformation behavior shown by different DP steels.