Model and validation of NbC nanoprecipitation during TMCP of X70 microalloyed steels

The effects of niobium (Nb) composition and temperature during the transition from rough rolling (RR) to finish rolling (FR) and during coiling were studied for NbC nanoprecipitation (<10 nm radius) in X70 microalloyed steels. Three (3) X70 microalloyed steels, with a composition of either 0.06 wt% Nb or 0.03 wt% Nb and/or with different cooling rates from rough rolling to finish rolling and/or different cooling interrupt temperatures (high (>550 °C) and low (<550 °C)), were studied. Matrix dissolution, in combination with transmission electron microscopy and X-ray diffraction, were used to quantify the size distribution and volume fraction of NbC nanoprecipitates in each steel. The measured peak radius and volume fraction of NbC nanoprecipitates for the 0.06 wt% Nb steels were 3.0–3.5 nm and 4.1 × 10−4, respectively, and 2.0 nm and 2.2 × 10−4 respectively, for the 0.03 wt% Nb steel. NbC precipitation was modeled using the PRISMA module of Thermo-Calc with the predicted size distributions showing good agreement with the measured NbC size distributions. For the processing conditions studied, model results showed that the majority of nano NbC precipitation occurred during finish rolling. The predicted size distribution of these precipitates was affected by the nominal Nb composition and the finish rolling temperature(s). Model results indicated that nucleation and growth of NbC in the ferrite (for the conditions studied) were negligible. A simulation with increased cooling rate between the end of rough rolling and the start of finish rolling showed a decrease in the size of NbC nanoprecipitates from 3.3 nm to 2.6 nm.