In the 1970s, anew refractory known as ferrosilicon nitride came on the market; it is produced from powder with a high ferrosilicon content that is saturated with nitrogen in a high-temperature resistance furnace. The ferroalloy nitrided in this way consists mainly (70-80 %) of silicon nitride Si3N4, together with iron and/or its silicides and impurities typical of standard ferrosilicon with 75 % Si. ferrosilicon nitride is intended for use as a strengthener in unshaped refractory mixtures. Tap-hole and gutter materials for blast-furnace production made with the addition of ferrosilicon nitride prove highly effective and are widely used, especially in Japan.
The ferrosilicon nitride content in unshaped refractories may vary widely (5-30 %), depending on the operating conditions. In particular, the size of the blast furnace and the conditions of hot-metal and slag discharge have a considerable influence on the composition of the tap-hole mud and channel materials. In all circumstances, however, the introduction of nitrided ferrosilicon increases the strength and wear resistance of the refractories at high temperature, increases their thermal stability and resistance to oxidation, and reduces the thermal-expansion coefficient, with simultaneous increase in thermal conductivity. It is also important that the refractories are more resistant to abrasion by the hot metal and the slag. In addition, the refractories do not shrink on repeated heating. Tap-hole mud with ferrosilicon nitride is most effective in closing the tap holes of furnaces with a large working space.
Somewhat later, ferrosilicon nitride began to be used for alloying in smelting corrosion-resistant steel with an elevated nitrogen content by electroslag remelting under pressure. Thanks to the high nitrogen content in the ferrosilicon nitride used for alloying (25-30 % N) and its relatively low carbon content, it is possible to smelt high-strength steel with the maximum quantity of nitrogen (more than 1.0 %). The consumption of alloying material is minimal here, with relatively high nitrogen assimilation by the melt. In comparison with the traditional manganese and chromium alloys used for alloying, the consumption of the ferrosilicon nitride is considerably less.
Despite the extensive positive experience with ferrosilicon nitride around the world, it is a new material for Russian plants. It has only begun to be introduced in Russia in the last few years, since the introduction of an industrial system for the manufacture of products from refractory inorganic compounds at LLC NTPF Etalon (Magnitogorsk). Relatively large-scale production (in amounts of a few tons) based on self-propagating high-temperature synthesis was introduced. In self-propagating high-temperature synthesis, besides such benefits as the absence of power consumption, the speed of the process at high pressure and maximum temperature, and the simplicity of the equipment, a new approach to the selection of the raw materials for synthesis is adopted here. Instead of the expensive pure-metal powders and scarce chemical reagents typical of self-propagating high-temperature synthesis, the new method employs ferroalloys, reducing agents, and other less expensive metallurgical products. For the first time, as a result, mass production based on self-propagating high-temperature synthesis is very economically efficient.
Note that, in comparison with conventional furnace technologies, self-propagating high-temperature synthesis yields higher-quality products, with a new combination of operating properties. For example, the use of vanadium-iron alloy, with a specified ratio of the components, has permitted the creation of a new alloy-fused nitrided ferrovanadium of high density (-6.5 g/cm3), with the maximum nitrogen content (10-12 %). It is impossible to obtain ingots of this material (of composition Fe-VN) by traditional furnace methods. The use of ferrovanadium nitride produced by self-propagating high-teraperature synthesis ensures practically complete assimilation of ihe nitrogen by the melt and guarantees a narrow concentration range of nitrogen and vanadium within the steel. The operational properties of the new materials are improved on account of the combination of high pressure of the reacting gas and high temperature of the process. Thus, in producing ferrosilicon nitride by self-propagating high-temperature synthesis, the temperature in the combustion zone is -2000 °C, while the pressure in the working space of the reactor is 10 MPa.
The introduction of ferrosilicon nitride produced by self-propagating high-temperature synthesis in Russia began with its successful use as an alloying additive in smelting transformer steel with nitride inhibition. In the converter shop at Magnitogorsk Steel Works (MMK), its use instead of nitrogen-bearing ferrochrome permitted practically 100 % increase in the nitrogen content of the steel. At LLC NTPF Etalon, a special combined method for alloying with nitrogen was developed for the production of nitrogen-bearing transformer steel. In the new method, the melt is preliminarily saturated with nitrogen, in the form of ferrosilicon-nitride pieces, in the ladle, in the production of steel with around 0.006% N. Finally, the nitrogen content is corrected by means of ferrosilicon nitride, which is preliminarily ground to powder, packed in a wire, and introduced during ladle treatment to obtain metal with 0.009-0.012 % N.
Furnace ferrosilicon nitride produced abroad usually contains around 30% N. The same material is used for alloying steel with nitrogen and for strengthening unshaped refractories. Ferrosilicon nitride produced by serf-propagating high-temperature synthesis (SHS ferrosilicon nitride), which is intended for the alloying of steel, contains less nitrogen: 18-23 %. Conversion to powder wire, whose filler contains less nitrogen, increases the assimilation of nitrogen and stabilizes the smelting of metal with narrow specified limits of nitrogen concentration. In addition, SHS ferrosilicon nitride compositions of higher density and strength have been specially developed for use in chunk form in alloying in the ladle. The density of such chunks is 1.5-2.0 times that of the traditional furnace product. The strength of the chunks is several times greater, and therefore dust formation may be almost completely eliminated when using the new material, with maximum increase in nitrogen assimilation by the steel.
Note that a simple technology has been developed for chunk alloying in the ladle; this technology not only ensures high and stable nitrogen assimilation in the steel but also permits an additional 5-10 % reduction in the total silicon consumption, on account of more complete silicon assimilation from ordinary ferrosilicon.
In contrast to the SHS ferrosilicon nitride used to alloy steel, the nitrogen concentration is higher in compositions used as additives in unshaped refractories. LLC NTPF Etalon has developed ferrosilicon nitride with an increased nitrogen content and reduced iron concentration. A higher silicon-nitride concentration is ensured both by appropriate selection of the raw material and by choosing conditions that ensure a product with the maximum nitrogen content. Investigation of tap-hole and channel refractories containing SHS ferrosilicon nitride shows that the ferrosilicon nitride content must be at least 75 %. After industrial tests in 2006, the MMK blast-furnace shop has now completely converted to water-free refractories containing SHS ferrosilicon nitride. The introduction of the new refractories is associated with more prolonged hot-metal discharge and a greater discharge of smelting products. In addition, working conditions for the blast-furnace staff are much improved. Note that the properties of tap-hole refractories with SHS ferrosilicon nitride are better not only than those of traditional nitride-free refractories but those of materials containing ordinary foreign furnace ferrosilicon nitride.
The SHS ferrositicon nitride is based on silicon nitride, which is a source of nitrogen when using the material as an alloying additive. If ferrosilicon nitride is introduced in refractories, the Si3N4 has a strengthening effect. Pure silicon nitride contains almost 40 % nitrogen (the stoichiometric content is 39.94 % N). It is practically inhomogeneous. At normal pressure (0.1 MPa), silicon nitride breaks down without melting at ~1900 °C. Raising the pressure increases the temperature stability of Si3N4. On contact with steel melt, the silicon nitride actively reacts, releasing nitrogen. The formation of silicon nitride is accompanied by the liberation of large quantities of heat
3Si + 2N2 — Si3N4 + 75.18 kJ/mol.
Because the reaction is highly exothermal, the synthesis of silicon nitride is possible in self-sustaining combustion. The theoretical adiabatic temperature of silicon combustion In nitrogen is exceptionally high-more than 4000 °C. However, this corresponds to the assumptions that all the silicon is converted to nitride and that there are no heat losses to the surroundings. It is also assumed that the products of synthesis do not sublimate. In practice, these conditions are very difficult to reproduce. Therefore, the experimental maximum temperature when silicon burns in nitrogen is much less: 1900-2200 °C.
The adiabatic temperature of ferrosilicon combustion in nitrogen will be lower when the theoretical combustion temperature of silicon is lower, since the thermal effect of the reaction between the alloy and nitrogen is known to be less than the exothermal effect of the pure metal. This is mainly because the ferrosilicon contains a considerable quantity of iron, which, in contrast with silicon, reacts with the nitrogen practically without heat liberation, while the nitrides formed here are thermally unstable. Another factor is that silicon and iron are bound in thermally stable silicides, whose decomposition requires considerable energy consumption. The combustion temperature is calculated from the conditions of equal enthalpies of the initial materials at the initial temperature (Tо) and the products of synthesis at the combustion temperature (Tc). Thus, all the heat liberated in Si3N4 formation is consumed in heating of the products, i.e., silicon nitride and iron
μ[H(Tг) - H(To)]Si3N4 + (1 - μ)[H(Tг) - H(To)]Fe = μQ,
where Q is the thermal effect of Si3N4 formation; μ is the proportion of silicon nitride in the product; H(Tо,), H(Тc) are the enthalpies of the combustion products at Tо and Tc
The combustion temperature calculated by this formula is relatively high for alloys with different Si content. Thus, even for ferrosilicon with 45 % Si, the theoretical combustion temperature is ~2500 °C. Hence, certain thermodynamic preconditions must be satisfied for successful self-propagating high-temperature synthesis in the ferrosilicon-nitrogen system over a broad range of initial alloy composition. In fact, experiments confirm that combustion may occur for all ferroalloys containing more than 40 % Si. In many ways, the combustion of ferrosilicon in nitrogen is similar to the combustion of metallic silicon. Thus, in nitriding, it is found that the combustion products contain considerable unreacted silicon, and consequently the combustion temperature is relatively low. Such incomplete conversion of silicon to the nitride is due to the low melting point of the silicon itself (1415 °C) and the relatively low dissociation temperature of its nitride. The conversion of silicon to the nitride in the experiments is 50-60 %. The melting point of Fe-Si alloys is even lower than for silicon. In alloys with 40-80 % Si, the liquid phase is formed above 1210 °C. Hence, processes associated with melting of the initial material in fenosilicon combustion may be more pronounced. Ferrosilicon nitride NITRO-FESIL®, produced by LLC "NTPF "Etalon":