An influence of the chemical composition on phase transformation regularities of martensitic stainless steels based on 13 mass % chromium with carbon content (0.04-0.1 mass %), extra alloying nickel (2.0-5.2 mass %) and molybdenum (0-1.20 mass %) has been investigated by calculated (with application of «Thermo Calc» program) and experimental approaches. An influence of nickel and chromium equivalents of investigated steel chemical composition on crystallization’s types (peritectical or single-phase mechanism with δ-ferrite formation), phase transformation temperatures and existence regions of different phases (δ-ferrite, γ-austenite, α-ferrite) has been identified. An increase in the concentration of ferrite-forming elements over the values of the chrome equivalent up to 16 mass % or more leads to the transition of steel to the martensitic-ferritic class. An increase in the content of austenite-forming elements primarily nickel reduces the lower border of the temperature range of the inverse α γ transformation and leads to the formation of stabilized austenite in the microstructure. Results of the phase transformation simulation have been compared with the microstructure and phase composition of two grades of industrial steel. It is established that the chemical inhomogeneity of the two-phase δ+γ structure formed during crystallization is preserved at room temperature.The heat treatment regimes connected with heating in the lower half of the two-phase α+γ region have been determined, when stabilized austenite is fixed in the steel structure at room temperature which affects its mechanical properties.The conducted research contributed to the development of new steel compositions at TMK Group plants for the production of seamless casing and tubing pipes of P110 13Cr steel grade, resistant to carbon dioxide corrosion, including operation in cold macroclimatic conditions.