Development and research of forming the properties in cold resistant steels with strength class not less than 950 MPa for the components of heavy carrying and lifting machines

The paper presents new high-strength steels for the components of heavy carrying and lifting machines, which operates in the Far North and Arctic areas. Combination of the main parameters of the developing steels leaves behind the existing global analogues and demonstrates high strength, ductility as well as cold resistance at the temperature down to –70 °С: tensile strength not less than 1,200 MPa, yield strength not less than 950 MPa, relative elongation not less than 10 %, hardness not less than 350 HBW, impact strength KСV-70 not less than 30 J/cm2. 4 steel chemical compositions on the base of С–Mn–Mo alloying system with additives of nickel and copper as well as microalloying elements, which were used together or separately, are examined in this research. Thermokinetic diagrams of decomposition of overcooled austenite were built and temperature-temporal conditions of forming of bainitemartensite structure were determined; these conditions are related for the new high-strength cold-resistant steels containing 0.17–0.21 % C; 0.70–1.30 % Mn; 0.25–0.35 % Si and 0.28–2.00 % (Ni+Cu+Mo). Influence of varying chemical composition on phase transformations and structure during continuous cooling of the examined steels was established and the heat treatment procedure for achievement of the required construction strength (combination of high tensile strength and low temperature impact strength) was suggested. As a result, it was determined that the most wide range of cooling rate values for obtaining of bainite-martensite structure (not less than 10 °С/с) and the optimal complex of strength and toughness-ductile properties are provided in the steel with the following chemical composition (mass. %): 0.21 С; 0.30 Si; 0.73 Mn; 0.017 Cr; 2.00 (Ni+Cu+Mo); 0.027 Ti; 0.003 B after quenching from 860 °С. The research was carried out by Nosov Magnitogorsk State Technical University under financial support of the grant of Russian Scientific Fund No. 13-19-20018 dated 20.04.2023, as well as of the remedies of the Ministry of Education and Science of Chelyabinsk region (Agreement No. 164 dated 28.06.2024), https://rscf.ru/project/23-19-20018/.