An experimental high-strength steel 20Kh2G2SNMA with high resistance of supercooled austenite to formation of diffusion transformation products is studied. Dilatometric analysis of the phase and structural transformations under continuous cooling and isothermal holding is performed. The time-temperature ranges of the transformations are found. The thermokinetic diagram is plotted. The mechanical properties of the steel are determined after different treatments, i.e., annealing, normalizing, quenching, tempering, isothermal and step quenching. It is shown that the presence of bainite in the structure lowers the impact toughness of the steel independently of its morphology and of the temperature range of its formation.