The paper presents stress-strain characteristics recorded during the four-step compression of axisymmetric samples in the Gleeble thermomechanical simulator. The hot deformability of three steels with Mn concentrations of 3%, 4% and 5% was compared. The analysis of the influence of plastic deformation and Mn content on the microstructure of alloys, and in particular, on a fraction and morphological features of the retained austenite, was performed. The proportion of the retained austenite was determined by the X-ray diffraction method. It was found that the content of Mn in the range from 3% to 5% does not have a significant impact on the high-temperature resistance of the steel during compression tests, but it has a significant influence on the microstructure of the steel and the fraction of retained austenite. The optimal conditions for maximizing the proportion of retained austenite were obtained at the temperature of 400 °C, and it decreased with increasing Mn concentration in the steel. It has been shown that it is related to the redistribution of carbon from the remaining austenite fraction with an increase in the manganese content. The mechanical properties were determined on the basis of hardness measurements.