The features of the formation of the structure of 4Kh4N5M4F2 (RATE Steel) steel with a change in the tempering temperature have been studied by methods of X-ray phase and X-ray structural analysis. Steel hardening is carried out from a temperature of 1110 $\pm$ 5°C, and tempering—in the range from 180 to 650°C with air cooling. The dependence of the change in the parameter of the crystal cell of martensite on the tempering temperature has been established. Changes in phase relationships in solid solutions ‘iron–carbon’ and carbide phases with an increase in tempering temperature have been determined. An interpretation of the relationship between the specified features of the crystal structure of steel and its physical and mechanical properties is proposed, where the maximum value of the parameter $a$ of the crystal cell reflects the maximum saturation of the $\alpha$-solid solution, which contributes to an increase in the resistance of the crystal lattice to deformation with an increase in resistivity, an increase in the hardness of martensite, a decrease in the toughness and an increase in its brittleness in the tempering temperature range (450–500°C) of the investigated steel 4Kh4N5M4F2. This explains the high values of the thermal conductivity of steel in the tempering temperature range of 450–500°C, which is associated with the retention of the amount of low-temperature carbides and an increase in the amount of high-temperature carbides with increased hardness. The maximum value of the parameter $a$ of the crystal cell is achieved at a temperature of 475°C. Such high values can be explained by the formation of a substitutional solid solution.