Vol 48, No 3 (2018)

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.

Fe–Ni alloys are widely used in engineering today. They are sometimes alloyed with boron. Oxygen is a harmful impurity in Fe–Ni alloys. It may be present in dissolved form or as nonmetallic inclusions. The presence of oxygen in Fe–Ni alloys impairs their performance. Research on the thermodynamics of oxygen solutions in Fe–Ni melts containing boron is of considerable interest in order to improve alloy production. The present work offers a thermodynamic analysis of solutions of oxygen in Fe–Ni melts containing boron. The equilibrium constant of the reaction between boron and oxygen dissolved in the melt in such systems is determined. The activity coefficients at infinite dilution and the interaction parameters in melts of different composition are also calculated. When boron reacts with oxygen in Fe–Ni melts, the oxide phase contains not only B₂O₃ but also FeO and NiO. The mole fractions of B₂O₃, FeO, and NiO in the oxide phase are calculated for different boron concentrations in Fe–Ni melts at 1873 K. For iron melts with low boron content, the mole fraction of boron oxide is ~0.1. With increase in the nickel and boron content in the melts, the boron-oxide content in the oxide phase increases. Its mole fraction is close to one for pure nickel. The solubility of oxygen in Fe–Ni melts is calculated as a function of the nickel and boron content. The deoxidizing ability of the boron improve significantly with increase in nickel content in the melt. The curves of oxygen solubility in Fe‒Ni melts containing boron pass through a minimum, which is shifted to higher boron content with increase in nickel content in the melt. The boron content at the minima on the curves of oxygen solubility are determined, as well as the corresponding minimum oxygen concentrations.

The oxygen-converter production of steel is determined by processes in the converter’s reaction zone, which consists of primary and secondary regions. The primary region is the crater formed by the collision of a supersonic gas jet with the molten-metal surface. It is filled with metal droplets (diameter 0.1–2 mm). The surrounding secondary region consists of melt with an enormous quantity of gas bubbles (diameter 0.2–4 mm). The total surface area of the droplets and bubbles is four orders of magnitude greater than the surface of the quiescent melt. That indicates the important role of processes at phase boundaries in steel production. The structure of the reaction zone and the corresponding temperature distribution are studied by hot simulation, when the molten metal is blown by oxygen in a transparent quartz crucible. The transparent walls permit photographic and video recording of the processes in the crucible. Besides the temperature distribution, the hydrodynamics of the bath may be studied directly in the injection zone. The most unexpected result of hot simulation is the motion of the bubbles in the secondary region. They move normal to the crater surface. In other words, their motion is almost horizontal, rather than vertical, as in cold simulation in water. This may be attributed to nonuniformity of the melt’s surface tension, resulting in motion of the bubbles toward higher temperatures. In liquid with a temperature gradient, the surface tension will be different ahead of and behind the bubbles. The forces pushing the bubbles from behind are greater than the forces at the front. Accordingly, they move toward the region of lower surface tension. The nonuniformity of the surface tension is due to the temperature gradient (up to 1200°C within the secondary region) and the change in concentration of the melt components, especially oxygen. The surface tension of the ferrocarbon melt changes in a complex manner with increase in temperature. The surface tension rises on heating to 1550°C, but begins to decrease beyond 1550–1600°C. With decrease in carbon content in the melt, the maximum value of the surface tension increases. The motion of gas bubbles and other phases toward lower surface tension begins at the 1550°C isotherm, which is therefore the external boundary of the secondary region, separating it from the remainder of the bath. Within this boundary, the resultant vector of the surface forces pushes the gas bubbles and slag particles, together with the molten metal, horizontally toward the crater, at increasing speed. This determines the hydrodynamics of the smelting bath and the associated redistribution of oxygen over different parts of the bath and hence the refining process as a whole.

Operational experience with the 5000-m³ blast furnace 9 at the Krivorozhstal plant shows that nonuniform blast distribution over the tuyeres results in very nonuniform distribution of the theoretical combustion temperature, the yield of blast-furnace gas, and the total energy of the blast flux and flux of blast-furnace gas over the hearth perimeter. That, in turn, significantly affects the smoothness of furnace operation, its productivity, the coke consumption, and the quality of the hot metal produced. The influence of 1% change in the variation coefficients of the input blast parameters on the productivity and coke consumption is taken into account. The results may be used in factorial analysis of periods of blast-furnace operation with different parameters. The nonuniform distribution of the blast flow rate over the tuyeres has the greatest influence on furnace performance. Therefore, it is necessary to find means of ensuring uniform distribution of blast at constant temperature over the tuyeres.

Computer software is developed for predicting the basic thermal characteristics in the plasma treatment of wheel rims for locomotives: the temperature, cooling rates, heat-flux density, and thickness of the martensitic layer at the wheel’s working surface. To verify the validity of this software, the results obtained are compared with thermometer readings and metallurgical data for the thickness of the martensitic layer. Two new features of the thermophysical model are the specification of the heat-transfer parameters in the plasma flare so as to ensure strengthening of the metal at the wheel rim by micromelting; and the ability to use the predicted temperatures and cooling rates in precision estimates of the martensitic-layer thickness. Estimates of the short-term thermal stress at the rim’s working surface show that compressive stress predominates. That is consistent with experimental data regarding the residual stress in steel samples after plasma treatment. If the proposed computer model is used in determining optimal conditions for the plasma quenching of locomotive wheels, the expenditures of time and material resources will be less than in in purely empirical selection of the operating conditions.

The possibility of import substitution by using Russian precision alloys developed at Bardin Central Research Institute of Ferrous Metallurgy in medical applications is considered. In many cases, Russian products outperform their imported counterparts. In practice, import substitution will call for close cooperation of Russian producers of medical equipment with the developers and manufacturers of high-quality metals.