A number of structural-mechanical strain-hardening models was analyzed, they were experimentally approved on austenitic and ferritic-pearlitic steels at a successive reduction in the sizes of their structural elements by thermomechanical treatments. The effect of a crystalline lattice of the matrix phase on the evolution of the steel structure, from the grain and mesolevels to micro-and nanolevels, was examined. The influence of a Mode I crack on an enhanced martensite transformation near its tip was noticed, which caused a simultaneous increase in the strength and cyclic fracture toughness of austenitic steel.Keywords: strain-hardening models, steels, structural deformation levels, prefracture zone near the crack tip.Introduction. Controlled variation of strength, plasticity, and fracture toughness properties, widely used in making structural steels and alloys, in particular forming and hardening of modern structural elements, is based on known and developed structural-mechanical strain-hardening and fracture models for metallic materials [1][2][3][4][5][6][7].For predicting and improving the strength properties of several structural steels, this study is restricted to examination of deformation-stable materials that do not undergo phase transformations under plastic deformation, inherent in metastable materials, which make strain-hardening models much more sophisticated [4,7].Analytical strength-structure relations for deformation-stable materials include geometric parameters and properties of structural elements and vary with a reduction in the sizes of the latter, from the grain and mesolevels to micro-and nanolevels. Variation ranges of structural element sizes at each of the levels should be consistent with corresponding strain-hardening models that determine the mechanical properties of the material. In this connection, the formulation of the object of investigation would be preceded by a short presentation of the state of the problem on the material structure evolution mechanisms and basic characteristics of limiting structural element sizes, changing analytical strength-structure relations.State of the Problem and Formulation of the Object of Investigation. Structural-mechanical strainhardening models and the scope of their application to structural metals at a directed reduction in structural element sizes by thermomechanical treatments are analyzed below.Basic Relations, Describing Strain Hardening of the Material. The Ludvik-Hollomon equations are the most-used empirical expressions relating the true stress s to the true strain e at the macrolevel