The addition of boron to steel introduces the possibility of steel initially completely solidifying and then remelting in the interdendritic regions at very low carbon levels. For all carbon levels, a low melting Fe-B alloy exists as a liquid down to around 1100°C. This liquid is found in steel with as low as 0.0004% boron due to the boron segregation. The persistence of this liquid leads to hot shortness, breakouts, and other casting defects. Pseudo-binary phase diagrams for low boron concentrations have been developed using ThermoCalc software and were confirmed using experimental techniques.