2024 Volume 64 Issue 2 Pages 326-337
Simulated thermochemical controlled processing (TMCP) was performed on four microalloyed plate steels with Nb and Mo contents varying from 0.03 to 0.045 and 0.03 to 0.15 wt pct, respectively, to investigate influences of both processing and alloying on transformation behavior and microstructural evolution. Dilatometry was performed in situ in a Gleeble® 3500 during thermomechanical simulation to construct continuous cooling transformation (CCT) diagrams for all alloys. A range of cooling rates between 2°C/s and a target 100°C/s along with two deformation levels, −0.4 total true strain and −0.6 total true strain in the austenite regime, were employed to create a range of microstructures. Increased deformation in the austenite non-recrystallization region promoted both polygonal ferrite and acicular ferrite transformation through an increase in nucleation sites. The increase in nucleation sites also resulted in a finer resultant microstructure with increased deformation. Increased cooling rates reduced transformation start temperatures and favored non-polygonal transformation products. Intermediate cooling rates led to the more desirable microstructures consisting of acicular ferrite and bainite. Both Nb and Mo increased the hardenability of the steel through interactions with the polygonal ferrite transformation. Nb and Mo retarded the polygonal ferrite transformation and favored an acicular morphology. Molybdenum alloying also favored bainite transformation. Desirable microstructures of acicular ferrite and bainite were able to be produced with the combination of higher deformation, intermediate cooling rates, and increased Nb/Mo alloying.
