Vol 63, No 7-8 (2019)

Nonmetallic inclusions in steel such as oxides, silicates, aluminates, spinels, etc. substantially reduce the quality of metal. They are mainly introduced by the slag formed on the metal surface of a doublestrand plant. It is proposed to separate the slag from the metal using a special perforated tube installed on the refractory pocket block of the ladle bottom and fixed with gunning mix. The use of this device for continuous slab casting ensures the high quality of hot- and cold-rolled sheets.

A modern pipe-rolling unit intended for the production of seamless pipes includes a continuous rolling mill with a retained mandrel. During operation, the working portion of the mandrel unit of a continuous rolling mill is subject to high temperatures and constant cyclic alternating loads, which results in its wear. The magnitude and nature of the wear of mandrels affect the quality of the inner surface of finished pipes. Also, a significant problem in the production of pipes on a continuous rolling mill is the short service life of mandrels. Moreover, mandrels are mostly imported and very expansive. In normal operating conditions, the amount of wear along the length of the mandrel is not uniform. The wear of mandrels can be made uniform and their service life can be increased by changing the velocity of the mandrel, changing the position of the mandrel at the beginning of the rolling process, or using mandrels of a new design.

In this article, the main developments and research aimed at increasing the service life of mandrels of FQM-type continuous rolling mills are described.

The “Kurchatov Institute” NRC, “Prometei” CRI of Structural Materials, in collaboration with the “OMZ-Spetsstal’” developed and mastered the technology for manufacturing long mandrels made of X35CrMoV05KU-UNI 2955 steel with guaranteed service life. The obtained mandrels meet the requirements of technical documentation as for their mechanical properties, hardness, grain sizes, ultrasonic testing, and contents of nonmetallic impurities. These mandrels passed through the industrial tests at the Taganrog Pipe Plant. The service life of the mandrels produced at the “OMZ-Spetsstal’” corresponds to the foreign analogs.

Results of studying the effect of the chemical and phase composition of non-metallic inclusions on the corrosion resistance of carbon and low-alloy steels in neutral aqueous media characteristic of the operating conditions of oilfield pipelines are presented. It is shown that the corrosivity of complex nonmetallic inclusions present in modern steels depends on their chemical and phase composition, the optimization of which can prevent an unfavorable effect of these inclusions on steel corrosion resistance under oilfield pipeline operating conditions. A method for reducing the corrosivity of non-metallic inclusions based on aluminum-magnesium spinel is its oxide component modification with calcium and/or titanium, which contribute to its conversion into a globular form. An increase in corrosivity of complex non-metallic inclusions, the oxide component of which consists of various components, and correspondingly a large amount of corundum among oxide components, results in a reduction of steel corrosion resistance. An increase in the content of calcium and magnesium oxides leads to a reduction in the corrosivity of complex inclusions. In producing steels with increased corrosion resistance for oil field pipelines it is advisable to provide production methods aimed at preventing formation of adverse types of non-metallic inclusions.

Bulk nanocomposite granules based on AlMg2 alloy, containing nanocrystalline graphite, are prepared by mechanical synthesis using a high-energy ball mill. Structural and phase compositions are studied by optical metallography, computer-aided X-ray tomography, Raman spectroscopy and transmission electron spectroscopy methods. A model of the structural formation is proposed for the granules produced at rotation rates of 660 and 815 min–1. Granule structure consists of slightly-misorientated aluminum solid solution sub-grains with sizes from tens to hundreds nanometers. It is proposed that the initial micro- size graphite particles are subjected to mechanical splitting during mechanical milling. Nanocrystalline graphite particles are located between aluminum grains, as well as at several grain junctions. These nanocomposite granules have high micro-hardness. The granular aluminum-matrix nano-composites may find application during component manufacture by pressure forming and as the precursors for preparing composite alloys by liquid-phase methods.

The structure and properties of the hot-extruded Ti–3Al–2.5V pipe 90.0 × 20.0 mm in cross section subjected to vacuum annealing are studied using methods of macro-, micro-, X-ray analyses, tensile and Vickers hardness tests. It is shown that during vacuum annealing within the microstructure there is development of α-phase and formation of a more equilibrium (α + β)-condition compared with the condition after hot extrusion, and these changes are reflected in phase parameters, their volume fraction and chemical composition. It is shown that development of recrystallization processes through a pipe cross section also play a specific role in forming the textural condition in different areas of the cross section (outer surface, 1/2 wall thickness, inner surface of the pipe). It is noted that vacuum annealing facilitates a reduction in the scatter of hardness in a pipe cross section and to formation of the combination of mechanical properties with good ductility and a moderate strength level.

Diffusion interaction at an interlayer boundary of St3 steel–AD1 aluminum after explosive welding and aluminizing by melt immersion is investigated. Test heat treatment regimes are revealed providing formation of coatings based on iron aluminides on a steel surface. It is shown that in order to form Fe₂Al₅ intermetallic on a steel surface with the hardness of 10 GPa, an explosion welded steel-aluminum composite must be given double heat treatment. After the first (660 °С, 3 h) a two-layer (Fe₂Al₅ and FeAl₃) diffusion zone is formed, and after the second (640 °С, 3 h) it is destroyed along a main crack between the Fe₂Al₅ and FeAl₃ interlayers. In order to form an Fe₂Al₅ intermetallic coating on a steel surface an aluminized layer obtained by melt immersion must be heat treated at 800 °C. Doping the diffusion zone with Si leads to the appearance of additional phases Al₇Fe₂Si and (Al, Si)₅Fe₃ within the composition, and an increase heat treatment duration leads to a reduction in coating hardness to 7.5–8 GPa.

The NLMK has introduced a computer-based training software system (CTS) for advanced training of the process personnel. The implementation of the system made it possible to improve the personnel’s skills to the level required to control the thermal condition of the blast furnace. The CTS allows studying the transient dynamics in the channels of control of the thermal condition of the furnace, recognizing trends in the thermal condition, and training the personnel’s kills of making adequate decisions on change in the thermal condition of the furnace. The system helped to reduce the amount of off-grade iron in blast furnaces.

A model has been developed for predicting the metal temperature in the specified superheat range above the liquidus temperature inside the tundish of a continuous casting machine (CCM) depending on the last temperature measurement in the steel-pouring ladle (prior to casting), as well as ladle history and secondary steelmaking process under the conditions of the Casting and Rolling Complex (CRC) of the JSC “Vyksa Metallurgical Plant” (VMZ). It is suggested to predict the superheat temperature of steel at the CRC by using statistical models considering the specifics of technological process. During the first stage, the model itself has been developed by integrating the following two approaches: a machinelearning algorithm and probabilistic graphical model (PGM and Bayesian networks).

The first approach provides the point estimate of the intermediate and final temperatures of the specific melting, while the second approach allows evaluating the probability distribution. The PGM-approach is very promising in the situations involving missing, incorrect, or undefined input data, which are quite common at the metallurgical plants. During the first stage, the achieved level of accuracy of tundish temperature prediction was 6.0°C.

During the second stage, the model was integrated into the shop automated process control system, which allowed increasing the prediction accuracy and ensuring a real-time control of the process parameters. The model is used as a guidance (master) tool for the process personnel operating the ladlefurnace and vacuum degasser equipment.

The behavior of vanadium in an existing production chain (with the study of conversion intermediate products) is studied using a set of modern analytical methods (X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray fluorescence analysis). The possibility of using a set of contemporary methods to predict the degree of vanadium extraction is demonstrated.

A study was performed to determine the influence of technological and design parameters of continuous casting of steel on reducing the thickness of the billet along the technological line of a curvilinear continuous casting machine (CCM) designed by Uralmashzavod OJSC. The effect of carbon content and casting speed on linear shrinkage through the billet’s thickness was investigated. It was established that in all cases there was a discrepancy between the thickness of the billet and the settings used for CCM rollers. This discrepancy can lead to defects in the billet caused by the distortion of its shape (owing to the blow-up under the action of ferrostatic pressure) and to an increase in friction and, accordingly, pulling forces. On the basis of the obtained results, new settings for the rollers on the hard and hydraulic sections were proposed. The use of recommended parameters considerably reduces the discrepancy between the thickness of the billet and the gap of the rollers, which will improve product quality.

We consider the operation of an existing cooling system for the rolls of a 1950 wide-strip hot-rolling mill aimed at the production of thin-sheet strips. On the basis of the results of computer simulation of this cooling system, we propose a new scheme for the supply of coolants. This system enables one to reduce thermal stresses on the surfaces of rollers, to improve the quality, and to decrease the thickness of hot-rolled thin-sheet steel from 1.3 mm to 1 mm.

The high-strength structure and properties of silicon-containing steel HY-TUF used in the aerospace industry and engineering for the manufacture of heavy-duty components is studied, including a dilatometric study of austenite transformation during continuous cooling in the range of constant cooling rates of 0.025–75°C/sec and during isothermal exposure in the bainitic range. The mechanical properties of HY-TUF steel after traditional quenching and tempering and isothermal quenching are determined. It is shown that during the isothermal exposure at 370–430°C and above low toughness upper bainite is formed, containing residual austenite with reduced carbon content. The relationship between the toughness of the bainitic structure and the reduction of area is shown, which is explained by the stability of residual austenite under load. It is substantiated scientifically that the best combination of toughness and strength of HY-TUF steel may be achieved after isothermal quenching in the lower bainite formation temperature range.

X-ray fluorescence analysis, X-ray powder diffraction, electron microprobe analysis, scanning electron microscopy, and thermal analysis are used to evaluate the material composition, structure and thermal properties of Dergamysh ore deposit and condensed roasted (750–850°C) products. Ore heating in air is accompanied by oxidation of sulfur and iron oxides, combustion and decomposition of sulfides, carbonate thermolysis, and decomposition of iron, copper, and zinc sulfates. Processes are generally completed up to 850°C. With desulfurization to the level of 60–94% sulfur the ore retains a porphyroblastic structure. Crystals (1–5 μm) of hexagonal and monoclinic pyrrhotites, corresponding to the general formula Fe0.80−0.93S, sphalerite and bornite border a mixture of fine (1 μm) olivine and spinel. In this case sulfides and oxides in contact with them achieve a similar concentration of non-ferrous metals, wt.%: up to 0.7 Co, 1.5–21.8 Cu, and 1.1–56.6 Zn. Heating a mixture of roasted (70–80% desulfurization) sulfide copper ore with nickeliferrous saprolite ore, calcium oxide and carbon in a weight ratio of 60:100:10:2.5 provides matte (4.1 wt. % Ni, 2.5 wt. % Cu, 0.38 wt. % Co, 2.1 g/ton Au, and 3.6 g/ton Ag) concentrating 90.3% nickel, 82.7% copper, 85.3% cobalt, and up to 99.0% of precious metals. The products of oxidation roasting of sulfide copper ores can be considered as an effective sulfiding agent and collector of valuable metals during smelting of nickeliferrous saprolite ores. The technology of joint reduction and sulfiding smelting of roasted copper ores and calcined saprolite ores is a promising way to use poor and mineral raw materials that are hard to enrich and to reduce SO2 emissions into the atmosphere.

The article considers conditions for nitrogen bubble formation at the crystallization front for iron and stainless steels. Using the results obtained during plasma-arc remelting, a semi-empirical formula is proposed for calculating maximum permissible nitrogen concentrations. It is shown that during steel crystallization with austenite formation at atmospheric pressure it is possible to obtain dense steel ingots with a nitrogen content exceeding its solubility at the liquidus temperature by a factor of 1.4–1.5.