Relation between the Degree of Alloying, Structure, and Mechanical Properties of High-Strength Steel

Development of the ocean, especially in Arctic regions, calls for the construction of an up-to-date fleet, including nuclear-powered icebreakers, arctic vessels, gas carriers, fixed and floating drilling platforms, submarine systems for oil and gas extraction on the continental shelf, reinforcement of coastal regions, and the construction of ports. That will require large quantities of weldable low-temperature steels that are also of high strength, so as to minimize the mass of the structures. The Zvezda shipbuilding complex in the far east of Russia is intended to meet that need. It is the largest such facility not only in Russia but in the world. In addition, the Vyborg ship-building plant and the Northern Shipyard (Severnaya Verf) in St Petersburg are being modernized. Another important task is the creation of new steels with the least possible alloying and standardized composition, so as to permit the development of more economical welding and assembly technologies. In the present work, the structure formed in low-alloy steels with variable content during plastic deformation is discussed. Samples from three melts of different chemical composition are studied: specifically, the melts differ in nickel content: 0.5, 1, and 2% Ni. The steels are tested on the Gleeble 3800 research complex, which simulates thermomechanical treatment with different temperatures in the final stage of rolling and with accelerated cooling to the specified temperature. The structure is studied by optical metallography and crystallographic analysis using a scanning electron microscope (EBSD analysis). The mechanical properties of the steels are determined. The thermal and deformational treatment of the steel must be selected in accordance with their level of alloying—that is, with the final structure of the steel (ferrite–bainite, bainite, or martensite–bainite). It is found that, in steel with ferrite–bainite structure, the best approach to strengthening is to create small-angle boundaries in the α phase during plastic deformation. Steel with bainitic structure does not undergo marked strengthening as a result of change in the deformation temperature during the final stage of thermomechanical treatment. For martensite–bainite structure, no treatment ensures the creation of additional small-angle boundaries. That may be associated with subsequent polymorphic transformation by a shear mechanism.