Microstructural evolutions of low carbon Nb/Mo-containing bainitic steels during high-temperature tempering

In this work, the effects of tempering at three high temperatures (660, 680 and 700 degrees C) for different holding times (5 min to 16 h) on the secondary hardening in Nb-bearing and Nb-Mo-bearing bainitic steel strips were investigated. It was found that with 0.3 wt% Mo addition in Nb-Mo-bearing bainitic steel strip, the peak hardening occurred with a significant increment in hardness (up to 40 HV) after approximately 10 min of tempering at all temperatures (660, 680 and 700 degrees C). In contrast, for the Nb-bearing steel strip, the corresponding increment in hardness was small (about 15 HV). The microstructural evolutions of Nb-bearing and NbMo-bearing steel strips during tempering at 700 degrees C were investigated with scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. It was found that during tempering, bainitic ferrite platelets in Nb-Mo-bearing steel strips remained much more stable and had a higher dislocation density than those in Nb-bearing steel strips. High-resolution transmission electron microscopy provided strong evidence that the Mo addition had significant effects on suppressing the annihilation rate of dislocations and retarding the coarsening rate of nanometer-sized carbides at higher temperatures, leading to a remarkable resistance to softening after secondary hardening.