Ultra-Fine Grained Dual-Phase Steels

This paper provides an overview on obtaining low-carbon ultra-fine grained dual-phase steels through rapidintercritical annealing of cold-rolled sheet as improved materials for automotive applications. a laboratoryprocessing route was designed that involves cold-rolling of a tempered martensite structure followed by asecond tempering step to produce a fine grained aggregate of ferrite and carbides as the initial microstructurefor rapid intercritical annealing. the intercritical annealing step was performed with heating and coolingrates of at least 100 °c/s and a holding time of 30 s. the intercritical temperature was selected to result in 20-35% martensite in the final microstructures for c-mn steels with carbon contents of 0.06, 0.12 and 0.17 wt%,respectively. the proposed processing routes produced an ultra-fine grained ferrite-martensite structure withgrain sizes of approximately 1 pm for all three steels. the tensile strength of these ultra-fine grained dual-phase steels can be increased by up to 200 mpa as compared to coarse-grained dual-phase steels whilemaintaining uniform elongation values. the rather narrow processing window necessary to obtain theseproperties was evaluated by determining the effect of intercritical annealing conditions on microstructureevolution. further, the experimental results were confirmed with phase field simulations of austeniteformation indicating that rapid heat treatment cycles are essential to obtain fine grained intercritical austenitethat leads to martensite islands with sizes of 1 pm and below in the final microstructure.