The strain hardening characteristics of low-carbon 10kp, 05G2S2, and 05G2R steels with a heterophase structure (ferritic-martensitic, ferritic-bainitic, ferritic-pearlitic) formed upon quenching from the intercritical temperature range and upon normalization are studied during uniaxial tension. The dislocation structures of these steels are examined by electron microscopy and internal friction, and the results obtained are used to explain the factors that cause the high strain-hardening rate of steels with a two-phase ferritic-martensitic structure. Stress-strain diagrams are constructed for steel samples that have various heterophase structures and are subjected to a small preliminary deformation (up to 10%); these diagrams can be used to compare the strain-hardening characteristics of the steels. © Pleiades Publishing, Ltd., 2009.