High manganese steels with different contents of carbon and additionally alloyed with silicon are investigated. Mechanical properties of the steels under tensile deformation are determined, and the changes in their phase compositions are studied. The phase compositions of steels after quenching and after quenching with subsequent deformation are investigated by the X-ray diffraction method. Magnetometric measurements are performed directly during tensile deformation. It is shown that deformation has a different effect on the phase composition of steels, namely: in steel 40G20, a small amount of strain martensite is formed as a result of deformation; in steel 25G20S3, a substantial part of austenite undergoes a martensitic transformation (γ → ε).