Study of phase and structural transformations in overcooled austenite for high-strength cold resistant steel during continuous cooling

Essential part of strategic objects in oil/gas and machine-building complexes, transport industry and bridge building are operated in the conditions of simultaneous effect of low temperatures, static and dynamic loads as well as intensive wear. High-strength cold-resistance steels are considered as one of the materials, which can be used in such conditions. At present time, providing of principally new qualitative combination of properties of such steels is the actual scientific problem; the main parameters of these steels should be superior to the existing international analogues, they should present the unique combination of high strength, ductility as well as wear and cold resistance at the temperatures down to -70 degrees C. However, improvement of strength properties and hardness of rolled products leads to substantial decrease of cold resistance and ductility of steel; so, reaching of combination between high strength state and cold resistance is possible during definite structural and phase transformations, occurring in steels. Regularities of phase and structural transformations, occurring in high-strength cold-resistant steel with the following chemical composition (%, mass.): 0.15 C; 1.5 Mn; 0.20 Si; 1.7 (Ni+Cu); 0.035 (Ti+V+Nb); 0.0040 B during continuous cooling, were established in this research. Thermokinetic diagram of decomposition of over-cooled austenite was built. It is shown, that steel with above-mentioned composition, which contains mainly if lathe martensite with small amount of bainite, is forming during continuous cooling with rate within the range 10-110 degrees C/c. Most part of martensite with lathe morphology can provide high strength of the researched steel, presence of dispersed structure of low-carbon bainite can provide high characteristics of resistance to brittle destruction, and hardness more than 350 HBW, which is achieved during cooling with rate more than 50 degrees C/c can provide high wear resistance.