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Dual phase steels are extensively used in automotive safety and structural components because of their low alloying costs, relatively easy processing and optimum combination of strength and ductility. This good combination of properties is attributed to the presence of soft ferrite phase and hard martensite islands in the microstructure. More specifically, the mechanical properties are dependent on the martensite volume fraction (increases strength), ferrite grain size (increases strength and ductility), martensite distribution and morphology (affecting UTS and ductility uniformity), and nano-precipitate characteristics (increases strength).
To increase strength in the dual-phase family then typically the martensite fraction is increased, but this comes as a cost to ductility. This paper reports on the use of a Rapid Alloy Processing (RAP) facility to systematically investigate different composition and process parameter modifications designed to improve the ductility / strength balance. The RAP route involves the production of cast ingots (around 8kg), thermo-mechanically processing and annealling followed by mechanical property and microstructure chatacterisation. In this research work, RAP produced DP 800 steel has been benchmarked with respect to full scale commercial product to confirm upscalability and alloy / process modelling and predictions have been made to propose DP variants with enhanced properties. This paper reports on a number of DP variants based on process / composition parameter changes, within the range constrained by commercial production, of a) heating rate variation during the continuous annealing cycle to modify ferrite recrystallization and austenite nucleation locations to promote the formation of isolated martensite islands, b) controlled coiling to generate a ferrite-bainite hot rolled microstructure followed by cold rolling and annealing to modify austenite nucleation and subsequent martensite distribution, c) modified composition (variations in Nb and Mn) to refine ferrite grain size without increasing hot rolled product strength, and d) modified composition (increased vanadium) to give precipitate hardening.