Theoretical and experimental studies of the mechanism of degradation of the structure of steels of main pipelines are carried out. It is shown that during long-term operation of pipes, the structure of the metal changes, its yield strength, hardness and relative elongation decrease. The formation of microcracks of critical dimensions with a decrease in plasticity leads to brittle fracture of the metal. Quantum-mechanical calculations of the structure of carbon steels made it possible to establish the value of carbon and the carbide phase in reducing the brittle strength of low-carbon steels at negative temperatures. Keywords: negative temperature, structural changes, decrease in plasticity, submicrocrack, crystal lattice parameter, brittle fracture, quantum mechanical calculations. uru.40@mail.ru