The article considers the effect of the newly designed Q-n-P-integrated («Quenching and Partitioning») heat treatment modes on the microstructure and mechanical properties changes of cost-saving alloyed high-strength steel 55Si3Mn2CrVMoNbА. It has been found that the application of Q-n-P-integrated technological schemes, with elongated soaking at 160°C, after previous bainitization, or thermal cycling, greatly increase the mechanical properties of 55Si3Mn2CrVMoNbА steel as compared to standard quenching and tempering (at 200-300°С). It has been found that the structure-free ferrite formation, contrary to presupposition, doesn’t increase the ductility of steel as compared to other investigated heat treatment modes. The most effective results showed the heat treatment according to TCT/Q-n-P mode (three cycles 160«250°С) which provided the formation in steel 55Si3Mn2CrVMoMbA high-strength state (TS = 1941-2116 MPa, 50-54 HRC) while maintaining high total elongation (10-16%) and impact toughness (KCU = 74 J/cm2), which results in a 5-fold increase in PSE («Product of Strength and Elongation») level (up to 31,5 GPа%) of treated steel as compared to quenching and tempering respectively. Heat treatment according to TCT/Q-n-P and В-Q-n-P modes in the investigated steel provides increased impact toughness level (up to 74-79 J/cm2) which makes it possible to use this steel with high cyclic and shock loads. Besides,according to the results shown in this article, the 55Si3Mn2CrVMoNbA steel properties in the state of Q-n-P-integrated treatment are in good agreement with the requirements for the 3rd generation high-strength steels. The study suggests that high mechanical properties are achieved due to the formation of the multiphase structure consisting of tempered martensite, lower bainite and significant amount of carbon enriched residual austenite. The obtained results demonstrate the promise of this technology for better mechanical properties of cost-saving alloyed machine-building and tool steels