24. Effects of Micro-Ti Addition on Improving Hot Ductility of Nb-Bearing Ultra High Strength Steels

Hot ductility of the continuous casting slabs of new generation of Nb-bearing steels (Ti-added steel #1 and Ti-free steel #2) has been studied using a Gleeble-3500 thermo-mechanical simulator. The influence of micro-Ti on hot ductility and the enhancing mechanism are investigated using OM, SEM and TEM. It is found that the brittle zone III is over the temperature range of 750–900 °C with a minimum reduction in area (RA) of 41.4% at 800 °C for steel #1, and of 650–950 °C with a minimum RA 19.4% at 750 °C for steel #2, respectively. Both steels have brittle zone III; however, the ductility trough of steel #2 is much deeper and wider than that of steel #1, which is mainly related to the film-like ferrite formation at the austenite grain boundaries. TEM morphology confirms Nb(C, N) precipitates at grain boundaries for steel #2. The precipitated particles strongly increase the crack sensitivity of the slab and result in hot ductility deterioration. In contrast, only (Ti, Nb)(C, N) particles are observed inside the grain for steel #1. Ti addition of 0.015 wt% can effectively promote (Ti, Nb)(C, N) to be nucleated on the prior TiN particles. The addition can greatly reduce the negative effect of grain boundary Nb(C, N) precipitations on hot ductility and significantly improve the hot ductility of the steel, thus making steel #1 has a much better hot workability than steel #2. Adding micro-Ti to Nb-bearing steel is an effective way to improve the hot ductility of the new generation of Nb-bearing steels.