INTERRELATIONS OF ALLOYING LEVEL, STRUCTURE AND MECHANICAL PROPERTIES OF HIGH-STRENGTH STEELS

Meeting the challenges of the world’s oceans development, especially in the Arctic regions, in the first place, it is absolutely necessary to build a modern fleet, nuclear icebreakers, Arctic cargo ships, gas carriers, stationary and floating drilling structures and offshore platforms, underwater complexes that provide oil and gas production on the continental shelf; to reconstruct coastal areas; and to build harbors, that require a large number of cold-resistant high-strength weldable steels to reduce metal consumption in engineering structures. That’s why the Russian government motivates construction of the shipbuilding complex “Zvezda” situated in the Far East (which is the largest national and world-wide shipyard). Vyborg Dockyard and Severnaya Verf (Northern Shipyard) in St. Petersburg are being modernized. The creation of new steels with minimum alloying and unified chemical composition to enable the development of more economical technologies for welding and assembling such unique vessels and marine technical structures is an urgent task. The paper deals with the issues of formation of the low-alloy steel structure with variable nickel content during the plastic deformation process. The specimens taken from three experimental melts of different chemical composition with varying nickel content (0.5  %, 1  %, and 2  %) were investigated. Selected steels were tested by means of Gleeble 3800 imitating thermomechanical treatment with various temperature parameters of the finishing rolling stage and accelerated cooling up to the predetermined temperature. Mechanical properties were determined. The paper presents results of structure examination by means of optical metallography as well as crystallographic analysis of microstructure using scanning electron microscopy (EBSD analysis). It is demonstrated that the scheme of thermal-deformation effect should depend on the alloying level, i.e. the final structure of steel (ferrite-bainitic, bainitic or martensite-bainitic). The most effective strengthening in steels with a ferrite-bainitic structure is obtained by formation of low-angle boundaries in the α-phase during the plastic deformation. Steels with bainitic structure are not likely to be significantly strengthened by changing of deformation temperature parameters at the finishing stage of thermomechanical treatment. Conditions providing the formation of additional low-angle boundaries were not found in martensite-bainitic steels, which might be the subsequent effect of polymorphic transformation by shearing.

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