The effects of the addition of titanium and boron on the austenite grain refinement in as-cast S45C carbon steel have been investigated and the results have been discussed based on an Fe–TiB₂ pseudo-binary phase diagram. The molar ratio of the added titanium and boron was fixed at 1 : 2 and the estimated molar percent of the added TiB₂ was varied from 0 to 0.5. The average austenite grain diameter decreased from 1900 to 250 μm as the TiB₂ addition increased from 0 to 0.2 mol%, when the cooling rate was 0.02 K/s. The austenite grain diameter, however, did not exhibit further decrease when the TiB₂ addition increased from 0.2 to 0.5 mol%. The lower limit grain diameter of 250 μm was very close to the secondary dendrite arm spacing, which was not affected by the addition of titanium and boron. When the cooling rate of the molten steel increased, the grain size and the dendrite arm spacing decreased. For all cooling rates, the lower limit grain size was very close to the secondary dendrite arm spacing. Metallographic observations revealed that one austenite grain included many dendrite arms when titanium and boron was not added, while with the addition of these elements one dendrite arm included several austenite grains having the dimension of the dendrite arm diameter. It was suggested that TiB₂ particles and other inclusions such as TiC and Fe₂B were formed in the inter-dendritic positions during and after solidification and they controlled the grain boundary migration in the inter-dendritic positions.