The paper presents the results of a study of structure formation during continuous cooling in low-alloy steel grade 09G2SA-A used in the production of large-sized forgings for power engineering. Based on the results of dilatometric analysis, studies of changes in the hardness and structure of steel after cooling at different rates, a thermokinetic diagram (TKD) was constructed for the decomposition of supercooled austenite. The TKD is the basic element for creating a heat treatment technology for large-sized forgings that provides uniform mechanical properties due to controlled management of structure formation processes in the entire volume of the hardened workpiece. The studies established the interval of cooling rates, which ensures the absence of such structural components as pearlite or high-temperature bainite in the steel structure after hardening to prevent their adverse effect on the resistance to brittle fracture of the steel. The materials of the article were presented at the International Scientific and Practical Conference "Advanced Engineering Technologies AET 2022" held in St. Petersburg on November 21–25, 2022.