A study of the high-strength silicon-containing steel HY-TUF applied in the aerospace industry and engineering for the heavy-duty parts is conducted. A dilatometric study of the austenite transformation during the continuous cooling at the constant cooling rates 0.025…75 °C/s and during the isothermal holding in the bainitic range is carried out. e mechanical properties of the HY-TUF steel after traditional quenching and tempering and isothermal quenching are determined. It is shown that during the isothermal holding at 370…430 °C the upper bainite with low toughness is formed, containing residual austenite with low carbon content. e relationship between the toughness of the bainitic structure and the reduction of area is shown, which is explained by the stability of residual austenite under load. It is scientifically justified that the best combination of toughness and strength of the HY-TUF steel may be achieved after isothermal quenching in the temperature range of the lower bainite formation