We studied the effect of boron on the hot ductility of micro-alloyed steels for physical simulation of continuous casting of steel conditions. For the physical simulation of the hot ductility of steel we used a thermomechanical Gleeble-3800complex. The paper identified temperature ranges of ductility failure in microalloyed steels with different boron compositions and under different conditions of cooling of the slab at the continuous casting of steel (continuous casting machine), ways to remove the ductility failures were also found. Joint microalloying of steel by boron, niobium and titan considerably increases hot ductility of steel in a temperature interval of 750-1050 °C. Positive influence of boron consists in modifying and crushing of initial grain of austenite. Modifying by boron is effective for increasing the hot ductility of steel under simultaneous microalloying of steel by titan and (or) niobium that allows to prevent formation of boron nitride and to eliminate its negative influence on ductility.