In the thermomechanical control process (TMCP) with accelerated cooling, the effect of the combined addition of boron with a grain-refining element, niobium, vanadium or titanium, on the mechanical properties and microstructures of low-carbon steels was studied. It was found that although the single addition of boron has little effect on the steel properties, the combined niobium-boron addition conspicuously improves the strength and toughness balance. In the case of the single addition of boron, coarse Fe₂₃(CB)₆ precipitates at γ-grain boundaries after rolling, and the γ-α transformation suppressing effect by boron is remarkably weakened. Niobium addition to boron steel has the effect of suppressing the precipitation of Fe₂₃(CB)₆ and strongly retards the γ-α transformation, and hence produces a fine-grained bainitic structure. Titanium has the same effect as that of niobium, but vanadium does not. The strengthening and toughening mechanism of steels by the combined niobium-boron addition was also discussed by examining the effect of alloying elements on the recrystallization stop temperature of γ during rolling, γ-α transformation behavior and so on.