The effect of the precipitation behavior of carbonitrides on austenite grain growth in Ti microalloyed steels has been studied. Transmission electron microscopy and optical microscopy were used to investigate the morphology, particle size distribution, composition and precipitation kinetics of carbonitride precipitates, as well as the austenite grain growth behavior. The main results and conclusions are as follows : The finer the size of carbonitride particles in an as-cast slab, the finer they were in the reheated condition. As a result, a higher GCT (Grain Coarsening Temperature) was exhibited by the Ti microalloyed steels. The steel air-cooled (A) after continuous casting exhibits much higher GCT than the steel quenched (B) after continuous casting at $\sim 1000^\circ C$, because of its higher volume fraction and smaller mean size of the carbonitride precipitates. The GCT of Ti-Nb steel is observed to be higher than that of the Ti microalloyed steel, the difference is, however, small. The Ti-Nb steel possesses coarser mean particle size and higher volume fraction of carbonitrides, as compared to Ti microalloyed steel. Therefore, this volume fraction difference of carbonitride precipitates is considered to be responsible for a slightly higher GCT in Ti-Nb microalloyed steels. We have calculated theoretical GCTs using Gladman's model in order to explain experimental observations. For the air-cooled steel A, the predicted GCT is in a very good agreement with the measured GCT. But, in the case of the quenched steel B, predicted GCT is about $100^\circ C$ higher than the measured GCT. This discrepancy mainly arises from the fact that the quenched steel B contains unusually small amount of carbonitride precipitates for the reason not clearly understood. A solution treatment of Ti microalloyed steels was possible at $1360^\circ C$. The GCT's of Ti microalloyed steels isothermally heat treated after a solution treatment observed to be below $1150^\circ C$. This is because these steels contain an unusually small volume fraction of carbonitride precipitates similarly to the quenched steel B. In addition, these solution treated and aged steel tend to contain excessively coarsened precipitates. The reason for this difference is not clearly determined at present. Precipitation(initiation) - Time - Temperature (PTT) curve has been determined using solution treated and aged steels. The nose of the C curve for the initiation of precipitation was observed at $\sim 1100^\circ C$ for $\sim$180sec.
