<p>X-ray diffractometry and scanning and transmission electron microscopy are used to investigate structural and phase transformations in low-activation chromium-manganese austenitic steel under long-term (500 h) aging at 700°C in quenched and cold-rolled conditions and its mechanical properties are determined. Particles of M₂₃C₆ precipitate after aging over the boundaries of grains and microtwins as well as inside grains on components of microstructural defects and fine particles of MC carbides. The aging lowers the strength properties of the steel. The post-aging structure of the quenched steel contains particles of a FeCrMn σ-phase, which reduces the elongation to 7% in tensile tests at room temperature to 7%. Brittle fracture components associated with the σ-phase appear on the fracture surfaces and are partially preserved at elevated test temperatures (600 – 700°C). The possibilities of adjusting the chemical composition and the treatment modes of novel low-activation austenitic steels aimed at prevention of formation of σ-phase under long-term aging in the operating temperature range (650 – 700°C) are discussed.</p>