Phenomenal Effect of Stable (Ti, Mo)C Nano-Sized Precipitates in Retarding the Recrystallization and Grain Growth in High-Strength Ferritic Steel

The present study demonstrates remarkable retardation of recrystallization and grain growth during sub-critical annealing of 60% cold-rolled ferritic steel containing Ti and Mo vis-a-vis a conventional microalloyed ferritic steel containing Nb and V. The evolution of Ti-Mo-C based clusters and nano-sized (Ti, Mo)C precipitates during the course of annealing in Ti-Mo added steel was studied extensively through transmission electron microscopy and atom probe tomography. The recrystallization kinetics was evaluated from the electron back-scattered diffraction analysis. The Ti-Mo added steel exhibited just a partially recrystallized (60%) fine ferrite grain structure (~ 8.8 µm) even after annealing for 24 h at 600°C. The conventional microalloyed ferritic steel, on the contrary, achieved almost complete recrystallization after only 4 h of annealing at the same temperature exhibiting a coarse grain structure (~ 18.3 µm). An intriguing aspect was the emergence of tiny partially-coherent (Ti, Mo)C precipitates in Ti-Mo steel after 8 h of annealing. Those precipitates effectively pinned down the dislocations and migrating ferrite boundaries, significantly retarding the recrystallization and grain growth, respectively. Grain refinement, substantial precipitation strengthening (up to 250 MPa) and strain-hardening from the partially-coherent (Ti, Mo)C nano-sized precipitates ensured an excellent combination of strength (UTS ~ 821 MPa) and ductility (~ 16.5% total elongation) in the 8 h annealed sample. Extensive yield point elongation (~ 4%) observed in that sample can be attributed to the combined effect of shearing of nano-sized partially-coherent precipitates by the dislocations along with Cottrell locking of dislocations by the solute atoms.