In this paper, we study the effect of cryogenic treatment on the microstructure and properties of high-strength D6AC steel with a bainite and martensite structure. The transformations occurring in steel during heat treatment above and below 0°C was dilatometrically studied. Cryogenic treatment in liquid nitrogen is shown to lead to almost complete restoration of the sample size, which indicates a decrease in the fraction of retained austenite in the structure. This is confirmed by the results of X-ray diffraction analysis. Cryogenic treatment is established to lead to increasing the strength properties and reducing the ductility of D6AC steel.