Isochronal phase transformation of Nb-V-Ti microalloyed ultra-high strength steel upon cooling

The transformation behavior of Nb-V-Ti microalloyed ultra-high strength steel during continuous cooling was investigated by Gleeble-1500 thermal mechanical simulator. Thermal dilation curves were measured at different cooling rates, based on which continuous cooling transformation (CCT) curve was established. The microstructure at slow cooling rate, primarily consists of polygonal ferrite, pearlite and lower bainite, while at intermediate cooling rate besides dominated lower bainite, small amount of polygonal ferrite can also be observed. At higher cooling rate, martensite lath is obtained. The nano-sized precipitates at slow cooling rate include M23C6, M2C, M3C, rich-Nb MC and rich-Ti MC and at intermediate cooling rate and higher cooling rate, the type of precipitates were similar, M3C and two types of MC. At cooling rates ranging from 0.2 to 1 °C/s, Vickers hardness of the steel decreases significantly. While at intermediate and higher cooling rates, the Vickers hardness increases gradually with increasing cooling rate. In addition, the polygonal ferrite transformation start temperatures (Ar3) decreases with increasing cooling rate, which is related to the carbide precipitation.