Strengthening Mechanisms for Ti- and Nb-Ti-micro-alloyed High-strength Steels

The strengthening mechanisms of hot-rolled steels micro-alloyed with Ti (ST-TQ500) and Nb-Ti (NT-TQ500) were investigated by examining the microstructures of steels using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results revealed almost no differences in the solute solution strengthening and fine-grained strengthening of the two steels, whereas the contributions of precipitation strengthening and dislocation strengthening were different for ST-TQ500 and NT-TQ500. The measured precipitation strengthening effect of ST-TQ500 was 88 MPa higher than that of NT-TQ500; this difference was primarily attributed to the stronger precipitation effect of the Ti-containing nanoscale particles. The dislocation strengthening effect of ST-TQ500 was approximately 80 MPa lower than that of NT-TQ500. This is thought to be related to differences in deformation behavior during the finishing rolling stage; the inhibition of dynamic recrystallization from Nb in NT-TQ500 (Nb-Ti) may lead to higher density of dislocations in the microstructure.