The introduction of carbon, nitrogen, and hydrogen in steel is analyzed in terms of the electron structure, dislocation properties, hardening, and failure of the steel. The similarity and differences in the mechanical properties of the corresponding solution solutions are discussed in relation to the influence of these elements on the density of electron states at the Fermi level of iron and correspondingly on the concentration of free electrons. Carbon reduces the concentration of free electrons, while nitrogen and hydrogen have the opposite effect. Hence, the atomic interaction is changed: specifically, its covalent or metallic component will be intensified. The dislocation rate in deformation is analyzed in the approximation of mobile and immobile interstitial atoms. In the first case, the interstitial atoms obstruct dislocational slip; the mobility of the dislocations is determined by the binding enthalpy of the dislocations with impurity atoms. If the interstitial atoms may accompany dislocations, the atomic bond is locally changed in dislocational atmospheres. That affects the unit energy of the dislocations and the distance between them in the slip planes. On the basis of the research results, the significant similarity between the hydrogen brittleness of austenitic steel and the ductile–brittle transition in alloying with nitrogen is explained.