A promising martensitic steel with high hardenability was studied. In the range of cooling rates 0.1…30 °C/s the only one transformation was registered by the dilatometer starting at a temperature of M s = 355 ± 10 °C. The microstructure and mechanical properties of the studied steel were analyzed after various heat treatment modes: cooling from austenitizing temperature with various rates (from 0.02 to 20 °C/s), austempering at a temperature 280…380 °C, oil quenching and tempering at a temperature 200…600 °C. It is shown that a 1000-fold change in the cooling rate weakly affects the strength of the studied steel. The bainitic transformation in the steel under consideration occurred in the temperature range of the martensitic transformation, which led to a decrease in impact toughness compared to the fully martensitic microstructure both after slow cooling and austempering.