The results of experimental studies of the structure and phase composition of the base metal and metal of the heat affected zone (HAZ) of model samples of high-strength low-carbon steel Alform 620M in conditions of various cooling rates (W$_{6/5}$) are presented, using a set of research methods including light, scanning and transmission electron microscopies. The goal is to study the influence of the structural and phase composition of the heat affected zone metal on its physical and mechanical properties, depending on the technological parameters of arc welding (W$_{6/5}$=3°C/s, 12°C/s and 25°C/s) at all structural levels (from grain to dislocation), as well as conditions for further operation (external static and dynamic load). Structural-phase characteristics are investigated—grain and sub-grain structure size, phase separation distribution, features of brittle and ductile fracture zones, distribution patterns of dislocation density in HAZ metal models. Comprehensive studies have made it possible to evaluate the differential contribution of certain structural-phase components that are formed under various welding modes, in optimizing the most important for operating conditions of mechanical characteristics of welded joints. Studies have shown that the base metal of high-strength Alform 620M steel has a fine-grained bainite structure with homogeneously distributed dislocation density and a lack of fragile destruction, which provides a high level of mechanical properties (strength, plasticity) and crack resistance of the metal. In model steel samples, when the cooling rate increases from W$_{6/5}$=3°C/s to W$_{6/5}$=25°C/s, the structural composition of the metal changes with an increase in the amount of lower bainite and dispersion of the structural components with homogeneous distribution of dislocation density. The optimum mode of cooling (W$_{6/5}$=25°C/s) is revealed, which, in point of view of the structure and phase composition, provides the highest level of mechanical properties and crack resistance of the Alform 620M high-strength steel welded joints.