Possibility of application of isothermal quenching for increasing of constructive strength of thick-walled increased reliability pipes on the basis of studying of isothermal decay features of the overcooled austenite in the bainitic transformation range of temperatures is investigated in our work. It is established that the reason of increase of cold resistance investigated steels is inheritance of initial morphology of martensite by ferrite in two-phase area and formation of subfine-crystalline structure. It is defined that observance of conditions of formation uniform heterogeneous martensite-bainite structure in the conditions of continuous cooling allows to receive the complex of properties corresponding to group of durability of P110 according to the standard API 5CT/ISO 11960 on steel 20H1MFA.