A dilatometric study of the bainite transformation in 20Kh2G2SNMA steel during isothermal holding in the temperature range of 280-380 °C was carried out. The kinetics of the bainite transformation is determined, the values of the coefficients k, n of the Austin-Rickett equation are established. The fraction of retained austenite in the steel structure after isothermal hardening was estimated using X-ray diffraction analysis. It is shown that at 320 °C and higher the carbon content in retained austenite is in satisfactory agreement with the theoretically predicted values, and below 320 °C, there is a discrepancy associated with slower bainite transformation and carbon partitioning. A correlation has been established between the impact strength, the fraction of bainite and retained austenite, and the carbon content in the latter. It is shown that with an increase in the fraction of bainite during isothermal holding, the fraction of retained austenite in steel increases, and the carbon content in it decreases, which negatively affects the impact strength of steel.