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UBC Theses and Dissertations

Grain refinement in dual phase steels Mukherjee, Krishnendu


In this work, a potential production route of ultra fine grained dual phase (DP) steels was studied. Deformation induced ferrite transformation (DIFT) was applied in laboratory tests employing a Gleeble 3500 thermo-mechanical simulator to produce fine grained dual phase steels in two chemistries: a conventional DP 600 chemistry with 0.06 wt% C-1.9 wt% Mn-0.16 wt% Mo and the C-Mn base chemistry of 0.06 wt% C-1.8 wt% Mn with no Mo addition. This thermo-mechanical treatment consisted of cooling the steel from the austenitization temperature at a rate of 40°C/s to a deformation temperature, which was 25 to 50°C above the austenite to ferrite transformation start temperature (Ar3) specific for the given austenitization and cooling conditions. Then the steel was immediately deformed to a true strain of up to 0.7 followed by rapid quenching. The effects of prior austenite grain size, amount of strain and deformation temperature on DIFT microstructures were studied to identify the most suitable thermo-mechanical path to obtain an ultra fine grained dual phase structure. Microstructures were characterized by scanning electron microscopy (SEM) including electron back scatter diffraction (EBSD) mapping. For the investigated steels the highest amount of deformation with a true strain of 0.6 or above resulted in optimized microstructures consisting of 70-80% polygonal ferrite with a mean grain size of 1-2 μm. Simulation of DIFT hot rolling schedules were conducted with hot torsion tests to investigate the viability of the proposed approach. A two-dimensional phase field model was developed to describe the austenite to ferrite transformation during DIFT. Several nucleation schemes were examined in terms of time and position of forming ferrite nuclei in the austenite domain to replicate the experimentally observed ferrite grain size spread. The austenite-ferrite interface mobility was used as the adjustable parameter to match the experimentally observed ferrite fraction.

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