A theory of martensitic transformations of carbon steels based on the principle of detailed balance is proposed. This theory supplements the theory of Landau phase transitions in crystalline media, as well as the theory of crystallization of Kolmogorov’s and Avrami’s substance. In the proposed theory the martensitic transformations are described by the equations of induced transitions between two energy levels, which correspond to the number of particles of the initial phase with low energy (austenite) and the number of particles of the transformed phase with higher energy (martensite). The source of the transformations is the particles of the third kind – phonons, the number of which is associated with the induced transitions of particles between the energy levels and depends on the transformation temperature. The conclusions of the proposed theory correspond quantitatively to the experimental data.