<p>Based on statistical processing of experimental data and thermodynamic calculations, compositions of high strength steels with bainitic structure and improved impact toughness were proposed. The stability of undercooled austenite and the ratio of the impact toughness of lower bainite to that of martensite were used as criteria for developing the steel compositions. The relative impact toughness of lower bainite correlates with the value of ∆<i>T</i>₀, which characterizes the position of the <i>T′</i>₀ = f(%C) dependence on the equilibrium diagram for a&#xa0;given steel composition relative to the theoretical dependence for the Fe–C system. Experimental CCT (continuous cooling transformation) diagrams of undercooled austenite transformation in the developed steels were constructed. Analysis of dilatometry curves and microstructure of the steels after continuous cooling at rates of 0.1–30 °C/s allowed the determination of the quantitative ratio of structural constituents. The mechanical properties of martensitic and bainitic microstructures in the investigated steels were determined. A&#xa0;comprehensive criterion for the mechanical properties of engineering steels, including strength, plasticity, and impact toughness, was proposed.</p>