Microstructural features and precipitation behavior of Ti, Nb and V microalloyed steel during isothermal processing

Thermal simulations of Ti, Nb and V microalloyed steel were conducted using a thermomechanical simulator, and the microstructural evolution and the precipitation behavior during isothermal processing were analyzed. The results show that with increasing holding time, the microstructural constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. The maximum hardness is obtained for the specimen after isothermal holding for 5 s due to the martensite strengthening effect. The hardness of the specimen decreases after isothermal holding for 10 s, because the strengthening contribution of fine dispersed precipitates becomes weaker. The hardness values of the specimens increase and then remain high after isothermal holding for 60 and 600 s. This is attributed to the contribution of the interphase precipitation hardening to the hardness of the studied steel. The precipitates in the specimen are coarsened after isothermal holding for 3600 s, even though the coarsening is not remarkable. These precipitates are fcc (Ti, Nb)(N, C) particles and belong to the MX-type precipitates. The beneficial effects of precipitation strengthening are lost. The hardness decreases to a minimum hardness value due to the presence of large amounts of polygonal ferrite after isothermal holding for 3600 s. Relatively coarse precipitates are the primary origin of the hardness decrease.