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 pct cold-rolled ferritic steel containing Ti and Mo. 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 pct) fine ferrite grain structure (~ 8.8 µ m) even after annealing for 24 h at 873 K (600 °C). An intriguing aspect was the emergence of tiny partially coherent (Ti, Mo)C precipitates in Ti–Mo steel after 8 hours of annealing. Those precipitates effectively pinned down the dislocations and migrating ferrite boundaries, significantly retarding the recrystallization and grain growth, respectively. Grain refinement and substantial precipitation strengthening from the partially coherent (Ti, Mo)C nano-sized precipitates ensured a decent combination of strength (UTS ~ 821 MPa) and ductility (~ 16.5 pct total elongation) in the 8 hours annealed sample. Extensive yield point elongation (~ 4 pct) observed in that sample (undesired for automotive body application) 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.