Austenite grain coarsening under the influence of niobium carbonitrides

This work investigates grain growth behaviour under the influence of pinning carbonitrides in a niobium microalloyed steel. The effect of temperature and heating rate on the grain size is studied. The grain coarsening temperature is determined as a function of the heating rate. It is found that unpinning by precipitates occurs around 40-70 K below the temperature of complete dissolution of carbonitrides. It has been found that experimental results are best described assuming a random distribution of carbonitrides in the matrix and niobium volume diffusion as the rate controlling process for the coarsening of precipitates. Austenite grain growth is explained theoretically taking as a basis the model proposed by Zener, which has been adapted for non-equilibrium kinetics, taking into account the experimental evidence that during a continuous heating, the amount of microalloyed element in solid solution is altered and different from that predicted by the solubility product.