Abstract: The effect of titanium content on the refinement of austenite grain size in as-cast peritectic carbon steel was investigated by fast directional solidification experiments with simulating the solidification and growth of surface and subsurface austenite in continuously cast slabs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to analyze the size and distribution of Ti(C,N) precipitates during solidification. Based on these results, the pinning pressure of Ti(C,N) precipitates on the growth of coarse columnar grains (CCGs) was studied. The results show that the austenite microstructure of as-cast peritectic carbon steel is mainly composed of the regions of CCGs and fine columnar grains (FCGs). Increasing the content of titanium reduces the region and the short axis of the CCGs. When the content of titanium is 0.09wt%, there is no CCG region. Dispersed microscale particles will firstly form in the liquid, which will decrease the transition temperature from FCGs to CCGs. The chain-like nanoscale Ti(C,N) will precipitate with the decrease of the transition temperature. Furthermore, calculations shows that the refinement of the CCGs is caused by the pinning effect of Ti(C,N) precipitates.