Grain Refinement in Hot Rolled Dual Phase Steels 2006-01-1432

Currently, grain refinement is being discussed for steels and other materials to increase both strength and toughness. However, for the automotive industry, a good combination of strength and ductility is desired which, for example, dual phase (DP) steels provide. Thus, in the present work the role of ferrite grain refinement is investigated in dual-phase steels. Deformation Induced Ferrite Transformation (DIFT) technique has been applied to produce ferrite grain refinement in four low carbon steels where starting from a conventional DP 600 chemistry Nb and Mo additions were varied. In this thermomechanical processing technique, the steels have been rapidly cooled from an austenitization temperature to the deformation temperature (which is at least 25°C above the Ar₃ temperature), to produce highly undercooled austenite, followed by heavy deformation, and subsequently rapid cooling thereby facilitating transformation to fine grained ferrite with martensite and/ or bainite. The effects of austenitization temperature, deformation temperature, and amount of deformation and steel chemistry on the final microstructure of the steels have been studied with tests performed on a Gleeble 3500 thermomechanical simulator. For all investigated steels, the maximum ferrite grain refinement (ferrite grains with a mean grain size of 1-2 μm) has been observed at the highest amount of deformation employed with a true strain of 0.6 for austenitization temperature of 950°C. Comparing hardness values for DIFT-DP microstructures with those obtained from conventional coarse grain DP structures, a strength increase of 20-40% is projected.