The effect of cryogenic treatment on the microstructure and properties of D6AC high-strength steel with a bainite and martensite microstructure has been studied. A dilatometric study of transformations occurring in steel during heat treatment above and below 0 °C was carried out. It is shown that cryogenic treatment in liquid nitrogen leads to almost complete recovery of the sample dimensions, which indicates a decrease in the fraction of retained austenite in the steel’s microstructure. This is confirmed by the X-ray diffraction analysis. It has been established that cryogenic treatment leads to an increase in the strength properties and a decrease in the ductility of D6AC steel.