Grain Refinement in Dual-Phase Steels

Deformation-induced ferrite transformation (DIFT) was applied in laboratory tests to produce fine-grained dual-phase (DP) steels. Four different chemistries were investigated, starting from a conventional DP 600 chemistry of 0.06 wt pct C-1.9 wt pct Mn-0.16 wt pct Mo and subsequently varying Nb and Mo additions. For all investigated steels, ultrafine ferrite (UFF) with a grain size of 1 to 2 μm can be obtained when a sufficient amount of deformation (e.g., a true strain of 0.6 or above in axisymmetric compression) is applied to an austenite microstructure with a grain size in the range of 10 to 20 μm at 25 °C to 50 °C above the austenite-to-ferrite transformation start temperature (Ar ₃) characteristic for the given cooling condition. Rapid post-deformation cooling at rates of approximately 100 °C/s yields the desired UFF-martensite microstructure. Electron backscattered diffraction (EBSD) mapping reveals a high percentage (approximately 40 pct) of low-angle boundaries in these microstructures, except for the steel that is just microalloyed with Nb. The steel with the plain-carbon-base chemistry was subjected to hot torsion simulations of a hot strip rolling processing schedules that incorporate a DIFT pass after a conventional seven-stand finish mill schedule. Executing the DIFT pass at 650 °C to 675 °C produced an UFF microstructure, illustrating the potential for the design of novel thermomechanical processing paths to produce hot-rolled ultrafine DP steels.