South Ural State University professor and researcher, doctor of technical sciences Gennadiy Mikhailov, has done a lot with his research team for the formation of thermodynamic theory of deep refinement of steel and they were the first in the world of science to build a diagram of the complex deoxidation of steel. Today, they are widely both in teaching future metallurgists and in designing the processes of complex deoxidation of steel in Russian metallurgical companies. Fragments of the theory have found application in the development of technologies of alloyed, electrical, and corrosion resistant steel. Dean of the Mechanical and Technological Faculty and the head of the SUSU Department of Physical Chemistry told us about which factors affect the quality of smelted metal, how the chemical composition of quality steel is regulated, how modern technologies for its smelting are developed, and why metal is still an art.
– What are the basics of your theory?
– We were able to create a method to simulate the processes of metal refining*. The main thing here is understanding the processes of interaction between the components of still during its cooling and crystallization. At these stages, we must finalize the technologies of removing oxygen, sulfur, phosphorus, and other residual elements that are harmful for the metal. For example, at the temperature of liquid iron, the maximum amount of oxygen that can dissolve in liquid metal at a temperature of 1600 ° C is 0.3%.
If the concentration of oxygen exceeds the solutions, then the steel will be in equilibrium with molten oxides (FeO and Fe2O3). When iron begins to crystallize at a smelting temperature of higher than 1535 ° C, the iron crystals will grow, creating a network of solid metal. Oxides will stay liquid until a temperature of 1350-1400 ° C and, therefore, each particle of iron will end up in the oxide shell.
By its characteristics, this steel is close to the characteristics of oxides and will break down under the lightest load. So, the content of oxides in steel must be minimum. Crystallized steel must be a monolith of crystals of metallic iron. The technological operation of removing oxygen from metal is called detoxification of metal. The operation of removing sulfur is desulfurization, phosphorous – dephosphorization, nitrogen – denitrification, and hydrogen – dehydrogenation.
– Why is it necessary to complete the process of steel deoxidation?
– To smelt quality metals, you must remove harmful residual elements from it. Deoxidation is the process of reducing the amount of oxygen in a metal. This effect is achieved by introducing an insignificant quantity of residual elements which have high sensitivity to oxygen relative to iron. Manganese, silicon, aluminum, calcium, and other elements have such properties. Oxides of these elements, being lighter substances, are pushed to the top of the metal and assimilate as surface slag. As a result, the quantity of oxygen in the metal is significantly reduced. That is, the non-metallic inclusions which have come out as slag have pulled out oxygen, which is dangerous to the metal. The formation of the composition of metal deoxidizers is a big issue and requires a special approach for each group of steel grades. Until recently, choosing the composition of complex deoxidizers was done empirically. Now we can solve the issue of deep refining of steel by using calculation methods.
– What new things have you discovered studying the process of steel deoxidation?
– Alongside the research team of our department we were able to, by order of the former Chelyabinsk Metallurgical RDE, solve the issue of the rational composition of calcium alloys with silicon for the metal oxidation process. We proved that low-percent (10-12%) silico-calcium (SiCa) is just as effective as 30%. The thing is that 15-18% of 30% is not used, because it burns up under the surface of the steel. We also proved that for the purposes of deoxidation, it is sufficient to use low-percent silico-calcium, while for modification, that is, changing the composition of non-metallic inclusions and the structure of the metal, it is possible to use high-percent SiCa, which significantly reduces the consumption of calcium and improves work safety, minimizing the likelihood of silico-calcium gas explosions. By the way, after our work, corrections were made to the GOST on the deoxidation of steel using silico-calcium.
– What are the key points of the diagram system of the condition of steel deoxidation that you have developed?
– To receive quality metal, you don’t only have to remove oxygen, sulphur, phosphorous, and different harmful residual elements from it. You have to also introduce beneficial components – silicon, aluminum, chromium, nickel, and many more. And what will happen during their interaction with iron and the dissolved oxygen within it? What phases will they form in individual cases? These and other questions are answered by the special system of status diagrams developed by our faculty’s researchers, allowing for the determination of the composition of metals and its beneficial phases in equilibrium with other elements that are introduced. I will explain with an example: if we are deoxidizing using aluminum, then it turns out that given a specific composition of metals, hercynite - FeAl2O4 – will be in harmony with it, and with different compositions – corindon. Corindon is a very harmful non-metal inclusion, which negatively affects the quality of the steel because of its hardness, which is similar to diamond. Corundum formations lead to the development of cracks in the metal, and under fluctuating loads, it begins to break down. So, in the final operation of smelting, aluminum deoxidation is not allowed in any case.
The simultaneous use of aluminum and calcium allows for the significant quantity of oxide inclusions to be converted into aluminates and be removed, and the remaining particles of aluminates do not cause damage, and can partially have benefits.
Our system of diagrams will help in choosing the optimal combination of equilibriums of chemical substances and understand how metal will “behave” during smelting, which non-metal inclusions can develop, and how they will affect the quality of the steel.
– How does modern metallurgy differ from metallurgy 30 years ago?
– Not too long ago in ingots of fully killed steel, sinkholes began to form and metal losses were 20-25%. The technology of smelting was extremely wasteful: from 100 tons of good metal, after smelting, up to 30% of the smelted metal was top scrap and bottom discard. Today, the technology of smelting has been modernized, metals are no longer poured into individual ingots – the whole pouring process occurs within a continuous flow machine. Where earlier, melting in a furnace took 6-8 hours, this process has now been reduced to 50 minutes. In this case, metal yield was not 70-80% as before, but already 95-98%.
However, you should not think that modern equipment will replace people. After all, metallurgy is not just work – metallurgy is also an art! You must know the techniques of smelting well enough to make instant decisions in a fast pace process – should this or that component be added or not, should the smelting temperature be raised or lowered? Today we have learned to melt steel in which the oxygen concentration is just 0.0001%. Can you imagine what professionalism you need to smelt metal with such a tiny concentration of oxygen in the conditions of an open shop?! Today, steel must undergo the refining process, during which, at the very end, modifiers are added – substances which fortify the steel’s structure. From here, the metal’s uniformity is improved as well as the technical plasticity and strength. This treatment with chemicals, for example ferro-alloys with particular components, allow us to change the “face” of the steel. At this time, there are steel grades which contain dozens of active components. This metal does not only need to be smelted, but “felt”.
– How strong is the relationship, in your opinion, between chemistry and metallurgy?
– Metallurgy is chemistry – but the chemistry of high-temperature chemical processes. Combine alkali and acid and you get salt and water. For some reason, this reaction is a chemical process. However, let’s look at the technological process of obtaining titanium. We have a fairly high number of titanium magnetite ore deposits, but the difficult thing is that it is impossible to obtain titanium by completing a single, instant reaction. The metallurgy of titanium is a very long, multi-step process. This is the reason titanium is an expensive metal. First, the titanium ore is enriched to extract the ferrous titanate. Then the ferrous titanate is crushed and remedial smelting is completed. A slag is produced which contains 70-80% of TiO2 and low-concentration ferrotitanium (ferrotitanium cast iron). The cast iron goes to a different plant, while the slag from TiO2 is combined with a carbonaceous reducing agent and from this, briquettes are pressed which they then load into a hermetic electric furnace. Chlorine is fed into the furnace at a temperature of 800-900° C and, as a result, titanium tetrachloride is produced. TiSL4, at this temperature, exists in gaseous form. With cooling, TiCL4 turns into a liquid. The obtained TiCL4 is poured into a reactor, into which liquid magnesium is fed, and liquid MgCl2 oxides are obtained and titanium is created. But at this temperature, the titanium is hard and looks like a sponge with a pattern similar to the pattern we see in winter on windows. Further, with the help of argon gas, the liquid MgCl2 is extracted from the reactor, and only pure titanium remains. Then, the reactor is opened, the sponge is removed, pressed, and re-smelted in an electrovacuum arc furnace. MgCl2 goes to electrolysis, and as a result, they obtain the gas Cl2 and liquid magnesium. The chlorine is used for chlorination, and magnesium for recovery of TiCL4. And in this process, it is already impossible to separate what is traditional chemistry and what is metallurgy. The division of these is very conditional.