Oxygen-converter and electric steelmaking method of steel production

Technological progress has constantly changed the methods of obtaining steel. In the XIX century. And the first half of the XX century. Successively replaced each other Bessemer, Thomas and Martin processes. The introduction of the first two was determined by the composition of the ores and the iron obtained from them for conversion into steel. Emerged in the second half of the XIX century. The open-hearth method was universal, independent of the quality of pig iron and allowed to melt steel of various qualities (in the USSR it was the main one in the years of industrialization and still remains so in a number of Russian enterprises).

With the development of science, two processes proved to be the most effective in the steel-smelting division. In the oxygen-converter process, steel is produced from molten iron and scrap for 30-35 minutes instead of 6-8 hours for melting in an open-hearth furnace. In arc electric furnaces from scrap and cast iron, the melting process requires 50-70 min. Therefore in the middle of XX century. The rapid and wide introduction of the oxygen-converter method began. By 1997, its share in world steel production reached 60%. Marten furnaces now produce only 7% of the world’s steel, and they are quickly decommissioned.

The construction of oxygen-converter shops with the simultaneous dismantling of open-hearth furnaces requires large capital outlays. Therefore, even industrially developed countries with powerful metallurgy conducted reconstruction for a quarter of a century: Japan completed the transition to the converter method of steel production in the early 1970s, Germany, Great Britain and France – by the early 80’s, and the US only by the 90’s. In Russia and the PRC, it is still ongoing. The converter way radically changed the whole structure of the steel industry in the world as a whole and individual countries. In the USA, it accounts for 61% of the steel produced, in France – 64, Japan – 68. Britain – 74, Germany – 76, and in Luxembourg – 100%.

Electric steelmaking is the second most important process in steel production. Its development was promoted by relatively small costs even for large electric arc furnaces, their quick commissioning, and widespread use of scrap. The growth of obtaining electric steel was favored by the construction of many midi- and mini-plants. This led to economic benefits from the introduction of this process (the share of electric steel in the world is 33%). Significant influence is exerted by the magnitude of the cost of electricity, especially at HPPs. In the countries of the young ferrous metallurgy (Taiwan, the Republic of Korea, Brazil, etc.), electric steel accounts for 50 to 100% of the metal smelting, and in the main producing countries of steel (Japan, the USA, Western European countries) from 24 to 40% (Italy – 58%).

In the steelmaking industry, an eco-efficient method of continuous casting of steel was particularly important. His installations were first developed and implemented in the USSR and widely used in the world. They reduce production waste (“rubber”) by 20-30%, reducing the cost of their remelting. In 1995 this method spread 76% of all steel in the world. In Japan, France, Germany, Italy, in the continuous casting plant (UNRS), all the smelted steel was cast.

A new technology of revolutionary importance for the iron and steel industry is the production of steel directly from metallized pellets, bypassing the smelting of cast iron. The economic and environmental benefits of this process (direct reduction of iron – PVZH) are obvious. The rate of growth of production by the method of UWW is much higher than the blast furnace one: in 1995, 31 million tons of metal were produced in the world. Installations of the PVZ require a significant amount of energy (mainly natural gas). This encouraged them to be placed in fuel-surplus countries and regions. Asia accounts for 40% of the metal produced by this technology, South America -35%. In large producers of steel in North America, Western Europe, and also in Russia there were only a few pilot plants.

As in the production of cast iron, great changes have taken place in the world geography of obtaining steel. New technologies for the smelting of steel, especially in small enterprises, allowed them to be placed outside the old traditional centers and areas of the metallurgical industry of the developed countries of the world. They had a very strong influence on the creation of steelmaking enterprises in the newly industrialized countries, where they were built in areas that were not developed in industrial terms, often lacking primary raw materials for metallurgical production. So, a significant amount of steel (up to 2.5 million tons in 1995) gives Saudi Arabia.

During the period from 1950 to 1995, The main result of the shifts in the geography of the world steelmaking industry was its powerful growth in Eastern Europe and Asia. Their total share in the smelting of steel increased from 22 to 55%. However, the growth rate was less than in the receipt of pig iron in these regions, which is explained by the narrower market demand for steel in underdeveloped machine building. At the same time, the share of western regions fell more than half from 77% to 37%. In South America, Africa and Australia, smelting began to grow faster than cast iron: there were also large producers of metal. Significant shifts occurred in the acquisition of steel among the countries of the world. The long leadership of the United States ended in the mid-1970s, when the championship passed to the USSR and was retained by it until 1991. With the collapse of the USSR, Japan emerged ahead, and since 1997 – the PRC.

Rolling – the final (output) product of the final stage of the whole cycle of ferrous metallurgy. Its cost is 2-5 or more times higher than the cost of steel, it goes directly to consumers in all sectors of the national economy. Rolling is the main commodity of foreign trade in ferrous metallurgy products. World statistics does not give the cost parameters of the rolled products, limited only to its weight. Rolled products are very diverse, their composition – the range – in the countries of developed machine building reaches 20-30 thousand items and continues to grow and update depending on market requirements.

The main types of rolling production are as follows:

1) sheet metal (thin sheet is especially valuable up to 3 mm – up to 30-45% of all rolled products in different countries);
2) high-quality metal – round, shaped, etc. (10-30% of rolled products);
3) blanks for welded pipes and the pipes themselves – all-rolled, etc. (5-10%);
4) wire rod – hot-rolled wire (3-8%);
5) railway rolling – rails, etc. (4-5%).

At present, exceptional importance for a number of branches of machine building, primarily electronic, instrument-making and others, has acquired precision rolling, which is characterized by very high dimensional accuracy.

Pipe-rolling production in connection with the development of pipeline transport is very large: in the 90’s. Produced annually 66-70 million tons, of which 1/2 was welded (15% of a large diameter), and 1/3 – all-drawn. The main producers of pipes in 1990 – the USSR (more than one-fourth in the world), Japan (1/6), China, Germany and the United States. The creation of a powerful pipe production in the USSR was caused by the great need for them to pump oil and gas in the 1960s and 1970s. And the refusal of Western countries in the cold war to supply them to our country.

Scientific and technological progress has improved the quality of rolled metal (coating of other metals, plastics, varnishes), allowed the production of bent rolled sections, etc. This significantly improves the quality of products from rolled products.

Metallurgical rolling mills

Rolling mill is a machine for processing metals by pressure between rotating rollers. After the steelworkers molded the ingot, this huge bar began to be turned into products – into a car body, a railway rail or a construction beam. But for this it is necessary that the ingot accepts a form convenient for manufacturing details-either a long beam with a cross-section in the form of a square, a circle, a beam, or a steel sheet or wire, etc. These various ingot forms and accepts on rolling mills.

Rolling in a hot state was used only at the beginning of the XVIII century, and at first this way prepared more or less thin iron sheets, but already in 1769 began in this way to roll wire. The first rolling mill for iron dies was proposed by the English inventor Corte, when he was developing a method of puddling. Court first realized that in the manufacture of some products it is more rational to charge the hammer with only the squeezing of the slag, and the final form is given by rolling.

In 1783, Court received a patent on the method he had invented for rolling shaped iron with the help of special rollers. From the puddling kiln, the crimson came under the hammer, here it was forged and received the original shape, and then passed through the rollers. Then this method became very common.

However, it was not until the nineteenth century that the technique of rolling was put to the proper height, which was largely due to the intensive construction of railways. Then rolling mills were invented for the production of rails and wagon wheels, and then for many other operations.
The device rolling mill in the XIX century was simple. The rolls rotating in opposite directions grasped a white-hot metal strip and, squeezing more or less force, carried it between their surfaces. Thus, the metal of the article was subjected to a strong reduction at a high temperature and the workpiece acquired the required shape. In this case, for example, iron received properties that did not have by nature. Separate grains of metal, which before rolling were located in its mass in disorder, during the process of strong reduction were stretched and formed long fibers. Soft and brittle iron became after that elastic and durable.

By the end of the century, the rolling stock was so refined that not only solid but also hollow products began to be produced by this method. In 1885, the Mennesman brothers invented a method for rolling seamless iron pipes. Before this pipe had to be made from iron sheet – they were bent and welded. It was both long and expensive. At the Mensnemann camp, a round bar was passed between two obliquely placed rolls, acting on it in two ways. First, due to the frictional forces between the rollers and the workpiece, the latter began to rotate. Secondly, because of the shape of the rolls, the points of their average surface rotated faster than the extreme ones. Therefore, because of the oblique arrangement of the rolls, the workpiece was, as it were, screwed into the space between them. If the disc was solid, it would not be able to pass. But since it was preliminarily heated up to white heat, the metal of the billet began to twist and stretch, and in the axial zone it loosened – a cavity appeared that gradually spread along the entire length of the billet. Passing through the rolls, the workpiece was placed on a special rod (mandrel), due to which the inner cavity was given the correct circular cross-section. As a result, a thick-walled pipe emerged.

To reduce the thickness of the walls, the pipe was passed through a second so-called pilgrim rolling mill. He had two rolls of variable profile. When rolling the pipe, the distance between the rolls was first gradually reduced, and then made larger than the diameter of the pipe.

What is the device of modern rolling mills ingot usually passes through several rolling mills. The first one is blooming or slabbing. These are the most powerful rolling mills. They are called crimping, because their purpose is to squeeze the ingot, turn it into a long beam (bloom) or a plate (slab), from which then other products will be made on other mills.

Blooming and slabbing are gigantic machines. The productivity of modern blooming and slabbing – about 6 million tons of ingots per year, and the mass of ingots – from 1 to 18 tons.

Before cutting, the ingots must be heated well. They are kept from four to six hours in heating wells at 1100-1300 degrees Celsius. Then the ingots are removed from the crane and placed on an electric cart – an electric car, which feeds them to the blooming or slabbing.
Bloomberg has two huge swaths. The upper can rise and fall, reducing or enlarging the gap between itself and the lower roll.
The heated ingot, passing through the rolls, falls on the roller conveyor from the rotating rollers. The operator continuously changes the direction of rotation of the bloom rolls and rollers of the roller table. Therefore, the ingot moves through the rolls then forward and backward, and each time the operator increasingly reduces the gap between the rolls, increasingly compressing the ingot. After every 5-6 passes, a special mechanism – the tilt bar turns the ingot 90 degrees to process it from all sides. In the end, it turns out a long beam, which is guided along the roller table to the scissors. Here the beam is divided into pieces – bloom.

In the same way, rolling takes place on slabbing, with the only difference being that in the case of slabbing 4 rolls have 2 horizontal and 2 vertical rolls that process the ingot immediately from all sides. Then the resulting long plate is cut into flat slabs.

Blooming and sludge are used only in those factories where steel is cast in the old way – into ingots. Where continuous steel casting plants (UNRS) work, they get ready-made blouses or slabs.

Finished blouses and slabs go to other rolling mills where, as the metallurgists say, profiles or profile metal, that is, blanks of a certain thickness, shape, profile, are made of them from special rolling mills.

Sheet mills, rolling slabs in a sheet, have smooth rolls. On such rolls, you can not roll a rail or another product of a complex profile. Rolls, for example, rail-mill mills are cut-outs of the shape that is necessary to obtain the product. In each roll, half of the profile of the future product is cut out. When the rolls approach each other, it turns out, as the metallurgists say, a stream, or a caliber. There are several such calibers on each pair of rollers. The first has a form that is only remotely similar to the shape of the product, the next one is approaching it more and more, and finally, the last caliber exactly corresponds to the dimensions and shape of the product to be obtained. Steel is stubborn, and it must be deformed gradually, passing through all the calibers in turn. That is why most mills have more than one pair of rolls, and several. Frames with rolls (they are called cages) are installed: in parallel either in a row or in a staggered order. The hot workpiece rushes along the rollers from the stand to the crate, and in each stand it moves forward and backward, passing through all the calibers.

High-performance continuous-rolling mills are becoming increasingly common. Here stands stand in sequence one by one. Passing one cage, the workpiece falls into the second, third, fourth, etc. After each compression, the workpiece is stretched, and each subsequent crate must, over the same period of time, pass through the workpiece of an increasingly long length. Some continuous mills roll metal at a speed of 80 meters per second (290 kilometers per hour), and a year they process several million tons. For example, the productivity of a sheet wide-band continuous mill “2000”, working at the Novolipetsk Iron and Steel Works, reaches 6 million tons.

In the USSR, in the All-Union Scientific Research Institute of Metallurgical Engineering, fundamentally new casting-rolling mills were created. At them processes of continuous molding are combined in a uniform stream with a continuous rolling. Today dozens of such mills are working in our country for rolling steel, aluminum and copper wires. The need for pipes for the transportation of oil and natural gas over long distances necessitated the creation of pipe mills. The diameter of the oil and gas pipes has increased. The first pipelines were 0.2 meters in diameter, then they began to produce pipes of large diameters – up to 1.4 meters.

Two fundamentally different technologies for the production of pipes are used. The first way: the workpiece is heated to 1200-1300 degrees Celsius, and then on a special mill in it a hole is made (it is stitched) – a short tube (sleeve) with thick walls is produced. Then the sleeve is rolled into a long pipe. This is how seamless pipes are produced. The second way: the steel sheet or tape is rolled up into a pipe and welded in a straight line or in a spiral.

Large capacity is provided by continuous aggregates of butt-butt welding of pipes. This is a complex of dozens of machines and mechanisms operating in the same technological line. Here everything is automated: on the part of the operator controlling the complex, it remains only to press the buttons on the control panel. The process begins with the heating of a continuous steel strip.

Then the machines are folded into a pipe, welded along the seam, stretched out in length, reduced in diameter, calibrated, cut into parts, threaded. 500 meters of pipes every minute – this is the performance of the complex.

In recent years, a new direction has emerged: at the rolling mills, not preforms but immediately finished parts of machines are manufactured. On such mills, automotive and tractor semiaxes, spindles of textile spindles, parts of tractors, electric motors, drilling machines are rolled. Here the rolling replaced the labor-intensive operations: forging, stamping, pressing and machining on various metal cutting machines – turning, milling, planing, drilling, etc.

Metallurgical converters

In 1855, the Englishman Henry Bessemer conducted an interesting experiment: he melted a piece of blast furnace iron in the crucible and blew it with air. Fragile cast iron turned into malleable steel. All explained very simply – the oxygen of the air burned carbon from the melt, which was removed into the atmosphere in the form of oxide and dioxide. For the first time in the history of metallurgy, additional raw material heating was not required to produce the product. This is understandable, because Bessemer realized the exothermic reaction of burning carbon. The process was surprisingly fleeting. In a puddling furnace, steel was received only in a few hours, and here – in a matter of minutes. So Bessemer created a converter – an assembly that converts molten iron into steel without additional heating. Mendeleyev called the Bessemer converters furnaces without fuel. And since in shape the aggregate of Bessemer resembled a pear, he was called – the Bessemer “pear”.

In the Bessemer converter, not every cast iron can be melted, but only such that silicon and manganese are present in the composition. By connecting with the oxygen of the supplied air, they release a large amount of heat, which ensures a rapid burnout of carbon. Yet the heat is not enough to melt solid pieces of metal. Therefore, in the Bessemer converter, iron scrap or hard cast iron can not be processed.
This sharply limits the possibilities of its application.

The Bessemer process is a fast, cheap and simple way of getting steel, but there are also big drawbacks. As chemical reactions in the converter go very fast, carbon burns out, and harmful impurities – sulfur and phosphorus – remain in the steel and deteriorate its properties. In addition, when blowing, steel is saturated with nitrogen of air, and this worsens the metal. That is why, as soon as open-hearth furnaces appeared, the Bessemer converter was rarely used for steelmaking. Much more converters used for the smelting of non-ferrous metals – copper and nickel.

Today’s converter, of course, can in a certain sense be called a descendant of the Bessemer offspring, for in it, as before, steel is obtained by blowing liquid cast iron. But not by air, but by technically pure oxygen. It turned out to be much more effective.

The oxygen-converter method of steel smelting came to metallurgy more than half a century ago. Created in the Soviet Union on the proposal of the engineer-metallurgist N.I. Brain, he completely ousted the Bessemer process. And the world’s first ton of oxygen-converter steel was successfully smelted in 1936 at the Kiev plant Bolshevik.

It turned out that in this way it is possible not only to process liquid iron, but also to add significant amounts of solid cast iron and iron scrap, which previously could be processed only in open-hearth furnaces. That is why oxygen converters have become so widespread.
But it was not until the 1950s that the steelmaking converters finally came to the fore. The degree of heat utilization in the oxygen converter is much higher than in steelmaking units of the sub-type type. The thermal efficiency of the converter is 70 percent, and the open-hearth furnaces are not more than 30. In addition, the gases leaving the converter are used for afterburning in waste heat boilers, or as fuel in the removal of gases from the converter without afterburning.

There are three types of converters: bottom blowing, top and combined. Currently, the most common in the world are converters with an upper oxygen purge – the units are very efficient and relatively simple to operate. However, in recent years, all over the world converters with bottom and combined (top and bottom) blowers begin to press converters with an upper purge.

Let us consider an arrangement of an oxygen converter with an upper purge. The middle part of the body of the converter is cylindrical, the walls of the bath are spherical, the bottom is flat. The upper helical part is conical. The converter housing is made of steel sheets 30 – 90 mm thick. In converters with a cage up to 150 tons, the bottom is detachable, fix it to the body with bolts, which facilitates the repair work. With a cage of 250-350 tons, the converter is made hollow, which is caused by the need to create a rigid hull design that guarantees against breakthroughs of liquid metal.

The converter housing is fixed to a special support ring, to which trunnions are welded. One of the pins is connected through a gear coupling to the pivoting mechanism. In converters with a capacity of more than two hundred and fifty tons, both journals are driven. The converter is supported by trunnions on the bearings installed on the frames. The rotation mechanism allows you to rotate the converter around the horizontal axis.

The shell and the bottom of the converter are lined with refractory bricks. Oxygen supply to the bath of the converter for purging the metal is carried out through a special tuyere introduced into the converter neck.

The first operation of the converter process is the loading of scrap. The converter is tilted to some angle from the vertical axis and a special box-scoop of capacity through the neck is loaded into the converter scrap – iron and steel scrap. Usually, 20-25 percent of scrap is used for melting. If the scrap is not heated in the converter, then immediately fill the liquid cast iron. After this, the converter is placed in an upright position, an oxygen lance is injected through the neck into the converter.

To guide the slag into the converter, slag-forming materials are introduced through a special groove: lime and in a small amount iron ore and fluorspar.
After oxidizing the impurities of the iron and heating the metal to the specified values, the blowdown is stopped, the lance from the converter is removed and the metal and slag are poured into the buckets. Alloying additives and deoxidizers are introduced into the ladle.

The duration of melting in well-functioning converters is almost independent of their capacity and is 45 minutes, the duration of purging is 15-25 minutes. Each converter per month gives 800-1000 smelts. The stability of the converter is 600-800 smelts.
The movement of the metal in the converter is very complex, in addition to the oxygen jet, bubbles of carbon monoxide act on the liquid bath. The process of mixing is complicated by the fact that the slag is pushed by a stream of gas and the thickness of the metal and mixed with it. The movement of the bath and the expansion of the bath with precipitated carbon monoxide lead to a significant part of the liquid melt into an emulsion state in which the metal and slag droplets are closely intermixed. As a result, a large contact surface of the metal with the slag is created, which ensures high rates of oxidation of carbon.

Converters with bottom oxygen scavenging due to a smaller iron fume allow to obtain a larger (by 1.5-2 percent) yield of suitable steel compared to converters with an upper purge. The melting in the 180 ton converter with bottom blowing lasts 32-39 minutes, the blowing time is 12-14 minutes, that is, the output is higher than that of the converters with the upper purge. However, the need for an intermediate replacement of bottoms eliminates this difference in productivity.

The first converters with bottom blowdown abroad were built in 1966-1967. The need to create such a converter is mainly due to two reasons. First, the need to process cast irons with a high content of manganese, silicon and phosphorus. Since the redistribution of such cast iron in converters with an upper purge is accompanied by the ejection of metal during purging and does not ensure the proper stability of the chemical composition of the finished steel. Secondly, the fact that the converter with such a purge is the most acceptable design, allowing to carry out the reconstruction of existing Bessemer and Thomas workshops, and fits into the building of existing open-hearth shops. This converter is characterized by the presence of a large number of reaction zones, intensive oxidation of carbon from the first minutes of melting, and a low content of iron oxides in the slag. Due to the specifics of the operation of the steelmaking bath at bottom blowing in converters of this type, the yield is somewhat higher than in other converters, and the dust content of the exhaust gases is lower.

In bottom-bottomed converters with a large number of tuyeres, all technological processes are more intensive than in converters with an upper purge. However, the overall performance of bottom-blowing converters does not exceed significantly the same for top-purge converters due to the limited durability of the bottoms.

To protect the bottom of the converter from high temperatures, the lance is made in the form of two coaxial tubes – oxygen is supplied through the central one, and along the peripheral one – any hydrocarbon fuel, most often natural gas. Such tuyeres are usually 16-22. A large number of smaller tuyeres provides better mixing of the bath and a more relaxed melting stroke.

A jet of fuel separates the reaction zone from the bottom, lowers the temperature near the bottom at the outlet of the oxygen jets due to the selection of heat for fuel heating, cracking and dissociation of the constituent fuels and products of their oxidation. The cooling effect is also provided with pulverized lime, which is fed into the jet of oxygen. Thus, the purging of molten metal by several jets of oxygen from below creates a number of favorable features in the operation of the converter. A greater number of reaction zones and a large interfacial contact surface of the oxygen jets with the metal are provided. This makes it possible to increase the intensity of the purge, increase the rate of oxidation of carbon. Better mixing of the bath improves oxygen utilization. As a result, it becomes possible to melt large pieces of scrap by mass. The best hydrodynamics of the bath provides a more even and quiet course of all melting, virtually eliminating emissions. Because of this, in converters with bottom blowing, it is possible to process cast iron with an increased content of manganese and phosphorus.
The desire to increase the productivity of aggregates at the same time as the need to increase the homogeneity of the composition and temperature of the metal, when it is possible to manufacture steels of a wide range, led to the use of a combined blowdown with a relatively small amount (compared to only bottom blowing) of gases blown through the tuyeres installed in the bottom of the converter. Recently, two main versions of this process have appeared, when oxygen or inert gases are supplied from below to ensure intensive mixing of the bath and speed up the process of removing impurities. At the same time, as in the case of bottom blowing, dust lime can be fed from below along with the gases. For such an important indicator as the possible consumption of scrap, converters with top, bottom and combined blowdown are approximately at the same level, with a slightly higher yield suitable for bottom blowing.
At present, many different methods of combined purging of a molten bath, rationally combining upper and bottom blowing, are used and are being developed in the world, both oxygen and inert gases (argon, nitrogen) being used in the latter.

In an oxygen-converter process with an upper purge, sufficiently intensive mixing is achieved only in the middle of the melting with intensive oxidation of carbon. At the beginning and at the end of the melting, mixing is not enough, which makes it difficult to thoroughly refine the metal from sulfur and phosphorus. The combined supply of oxygen through the top and bottom tuyere even more than with one bottom purge accelerates the oxidation of carbon and increases the productivity of the converter.
Compared with pure bottom blowing in the case of a combined process under comparable conditions, the metal temperature is higher. In addition, with combined blowdown, a reduction in oxygen flow through the upper lance reduces dust formation and splashing.

And one more advantage of oxygen converters: here all processes are mechanized and automated; Increasingly, the management of converters is entrusted to computers.

Electric Arc Furnaces

The whole history of metallurgy is a struggle for quality, for improving the physical and mechanical properties of metal. And the key to quality is chemical purity. Even tiny admixtures of sulfur, phosphorus, arsenic, oxygen, some other elements sharply impair the strength and plasticity of the metal, make it brittle and weak. And all these impurities are in the ore and coke, and getting rid of them is difficult. During melting in a blast furnace and a Wartmann furnace, the bulk of the impurities is transferred to the slag and, together with it, is removed from the metal. But in the same blast furnaces and open hearths, harmful elements from combustible gases get into the metal and deteriorate its properties. To get really high-quality steel was helped by electrometallurgy, the branch of metallurgy, where metals and their alloys are produced by electric current. This applies not only to the smelting of steel, but also to the electrolyte of metals and, in particular, their molten salts – for example, the extraction of aluminum from molten alumina.

The bulk of alloyed high-quality steel is smelted in electric arc furnaces.

In arc steelmaking furnaces and plasma-arc furnaces (PWM), heat generation occurs due to energy transformations of an arc discharge occurring in air, vapor of melted materials, inert atmosphere or other plasma-forming medium.

According to the general theory of furnaces, MA. Glinkova arc steel-smelting and plasma-arc furnaces are heat exchangers with a radiation mode of operation, since the energy conditions at the boundary of the process zone, that is, on the mirror of the bath of liquid metal, create electric arcs and refractory lining of the working space. In addition, in arc steel furnaces, vertically arranged graphitized electrodes create uneven arc radiation, depending on the diameter of the electrodes and the parameters of the electrical regime.

According to the conditions of heat exchange between arcs, surfaces of working space and metal, features of electro-physical processes of arc discharge, energy and electrical modes, all melting in arc furnaces from the beginning of melting of solid metal charge to the discharge of liquid metal is divided into stages.
Before the beginning of melting, the domed roof of the furnace is lifted, sidetracked and charged on top of the furnace charge materials. Then the vault is put in place, the electrodes are lowered through the holes in it and the electric current is turned on. Cast iron, scrap iron and other materials begin to melt quickly.
As the burden is melted under the electrodes and around them, “wells” are formed, into which arcs and electrodes are lowered. There comes a stage of “closed” burning of arcs, when melting of the charge occurs in “wells”, from below by heat transfer to adjacent layers of the charge and heat conduction through a layer of liquid metal accumulated on the bottom. Cold charge on the periphery of the working space is heated due to the heat accumulated by the lining: the temperature of the inner surface of the liner decreases intensively from 1800-1900 to 900-1000 degrees Kelvin. At this stage, the lining of the working space is screened from the arc radiation, so it is advisable to provide the maximum thermal power taking into account the electrical capabilities of the furnace transformer.
When the amount of weld metal is sufficient to fill the voids between the pieces of solid charge, the electric arcs open and begin to burn above the mirror of the metal bath. There comes the stage of “open” burning of arcs, in which intense direct radiation of arcs occurs on the lining of walls and roof, the temperature rises at a speed of up to 30-100 degrees Kelvin per minute, and there arises the need to reduce the electric power of the arcs in accordance with the heat-sensing ability of the lining.

Modern arc steel-smelting furnaces operate on a three-phase current of industrial frequency. In arc furnaces of direct action, electric arcs arise between each of the three vertical graphitized electrodes and the metal. Lined casing in arc steelmaking furnaces has a spherical shape. The working space is covered with a dome vault from above. The casing is mounted on the support structure with a hydraulic (less often electromechanical) tilt mechanism. To drain the metal, tilt the oven 40-45 degrees, to download the slag – to 10-15 degrees (in the other direction). Furnaces are equipped with mechanisms for lifting and rotating the roof – to load the charge through the top of the furnace, moving the electrodes – to change the length of the arc and regulate the power input into the furnace. Large furnaces are equipped with devices for electromagnetic stirring of liquid metal in the bath, systems for removal and cleaning of furnace gases. Domestic plasma-arc furnaces have a capacity from 0.5 to 200 tons, power from 0.63 to 125 MW. Current strength on powerful and super-powerful plasma-arc furnaces reaches 50-100 kA.

Depending on the technological process and the composition of the slag, the lining of plasma arc furnaces can be acidic (when smelting steel for shaped casting) or the main (for smelting steel for ingots).

A feature of the design of plasma arc furnaces with refractory lining as a variety of melting furnaces for arc heating is the presence of one or more dc plasmatrons and an anode bottom electrode. To preserve the atmosphere of the plasma-forming gas, the working space of plasma-arc furnaces is sealed with special seals. The presence of a water-cooled electrode in the bottom creates a danger of explosion, therefore, plasma-arc furnaces are equipped with a monitoring system for the state of the lining of the bottom and signaling warning of penetration of the bottom electrode by liquid metal.
At present, plasma-arc furnaces with refractory lining with capacity from 0.25 to 30 tons with capacity from 0.2 to 25 MW are operating. The maximum current strength is up to 10 kA.

The most energy-intensive period of melting in furnaces of both types is the melting period. It is then consumed up to 80 percent of the total energy consumption, and mainly electric. The duration of all melting, depending on the technology adopted for melting electric steel, can be 1.5-5 hours. The electrical efficiency of arc steelmaking furnaces is 0.9-0.95, and the thermal efficiency is 0.65-0.7. Specific consumption of electric energy is 450-700 kW / h per ton, decreasing due to a decrease in the specific heat-dissipating surface for larger arc steel-smelting furnaces.

Plasma arc furnaces have lower values. Their electrical efficiency is 0.75-0.85. This is explained by the additional losses in the plasma torch during the formation of the plasma arc. Thermal – about 0.6, because there are additional losses in the water-cooled elements of the structure. A feature of the operation of plasma arc furnaces is the use of expensive plasma-forming gases, which necessitates the creation of systems for the regeneration of waste gases and the use of technologically acceptable cheap gas mixtures.

New opportunities in steelmaking appeared in connection with the successful development in the late 1980s of the bottom (through the bottom) of the release of metal from electric arc furnaces. Such a production system has been successfully implemented, for example, in the steelmaking shop of the company’s factory in Oberhausen (Germany), at the 100 tonne kilns of the plant in Friedrichsferke (Denmark), etc. They can operate continuously for a long time, for example, the Danish 100 ton Aggregates – within a week. At the release of the melting, which lasts no more than 2 minutes, the oven tilts only 10-15 degrees instead of 40-45 degrees (for conventional units). This allows almost completely replace the refractory lining of walls with water-cooled panels, dramatically reduce the consumption of various materials and electric power, and produce complete cutoff of furnace slag.

Surprisingly at first glance, the modern ultra-high power arc furnace has a specific energy consumption significantly lower than the open-hearth furnace. In addition, the work of the open-hearth furnace of the open-hearth furnace is much heavier and tedious than the work of a converter or electric steelmaker.

Blast furnace operation, air supply

Cast iron is smelted in special furnaces, called blast furnaces. Blast furnace is a huge tower (its height reaches 36 meters), consisting of two truncated cones, adjacent to wide bases to a low cylinder. Outside, the furnace is enclosed in a metal casing, inside it is lined with a special fire-resistant brick withstanding the high temperatures. In the upper part of the blast furnace – the top – there are pipes, through which gases are formed from the furnace; Here, there is also a loading device consisting of two alternately descending cones, which gently transfers the raw materials to the blast furnace, distributes it evenly and blocks the path escaping from the furnace gases. The mine is located behind the shaft. This is the largest part of the furnace, its height is 21 meters (therefore the blast furnace is referred to as mine type furnaces). The shaft rests on the opening – the cylindrical widest part of the furnace. The diameter of the blast furnace volume of 5,000 cubic meters is 16 meters. Then the blast furnace tapers again into a part called shoulder straps.

The lower part of the blast furnace – horn. The diameter of the furnace of a powerful blast furnace with a volume of 5000 cubic meters reaches 15 meters. This is the most important, if you can say so, the most loaded part of the blast furnace. Here there are holes – tuyeres, through which air is supplied. In the furnace, fuel is burned, liquid iron and slag accumulate in it. In this part of the furnace there are also discharge holes – tapholes (cast iron below, slag above, since slag is lighter). Therefore, the device of the furnace is given exceptional attention. The thickness of its walls reaches 1.5 meters. The whole blast furnace rests on a huge reinforced concrete foundation. Very high temperatures develop in the blast furnace. Against their impact, even the most fire-resistant refractory materials can not always stand. Therefore, the lower part of the furnace, where the temperature is higher, is cooled. In the refractory masonry are put metal plates – refrigerators, and in them are laid also metal tubes – coils on which water flows. A huge amount of fresh water is consumed to cool the blast furnace – up to 30 cubic meters when smelting 1 ton of pig iron is “drunk” by a modern blast furnace. Modern metallurgical plant spends more water than a city with a population of 100-200 thousand people.

Usually in a factory cooling water circulates through a closed system, i.e. for cooling use the same water. Near the blast-furnace shop hundreds of fountains are beaten-a spectacle not inferior to the fountains of Petrodvorets in beauty. It was splashing. The water, which absorbed the heat of the masonry of the blast furnace, is sprayed, cooled, collected in pools and again rushes to the aid of blast furnace. But the masonry of some blast furnaces is cooled not with cold water, but with steam, using evaporative cooling. Boiling water fed to refrigerators evaporates, and the heat necessary for this process is taken from the refractory masonry of the blast furnace, thereby cooling it. When evaporation of 1 kilogram of water from the cooled element of the furnace, 539 kilocalories of heat are taken away. The resulting steam is used in waste heat boilers.

Air supply to the furnace

The blast furnace is continuously loaded with agglomerate or other iron ore materials, fuel, fluxes – those three main “dishes” that we already know. These materials are always supplied in a certain proportion, the furnace is always filled with charge. At the same time, it contains up to 7,000 tons of materials – 120 railway wagons. The very configuration of the furnace facilitates the “propping up” of the charge. Yes, and the air supplied to the furnace for burning fuel, comes under pressure, and the burden, as it were, rests on the air cushion. Domna “inhales” a huge amount of air. And often it does not suit ordinary air, but requires oxygen enriched.

One ton of pig iron consumes from 2,500 to 3,500 cubic meters of air (up to 8,000 cubic meters per minute). Such a mass of cold air would cool the furnace, reducing its productivity, increasing fuel consumption, disrupting the normal flow of the process. To avoid these troubles, metallurgists heat the air before feeding it into the furnace to a temperature of 1200 ° C and higher in air heaters (or as they are called by the name of the inventor – the Cowperers). Air heaters are located next to the blast furnace. They are towers up to 50 meters high. Outside, like the blast furnace, the air heater is closed with a metal casing (its external diameter is 9 meters), from the inside it is divided into two parts: a combustion chamber and a part filled with a refractory nozzle (a finned surface of refractory material).

Burning fuel is burned in the combustion chamber. The resulting combustion products are passed through a refractory nozzle to which they give their heat. Gradually, the nozzle is heated. When it is heated to a sufficiently high temperature, the fuel supply is stopped. On the opposite side, powerful air blowers start to pump cold air into the air heater, which, passing through the hot nozzle, takes away its heat, heats up, leaves the air heater and goes to the huge annular air duct surrounding the blast furnace. It is called a tuyere belt. From here, by special branches (sleeves) through the tuyeres, the air is uniformly blown into the blast furnace.

Since it takes a certain amount of time to heat the nozzle of the air heater, several (three for one oven or seven for two) are installed for uninterrupted supply of hot air near each blast furnace. Some air heaters are still heated, while others are already working (they heat the air). The air for cooling the blast furnace is taken directly from the atmosphere. Its humidity throughout the year varies within very wide limits, and this adversely affects the operation of the furnace. It rains or snow, and too much moisture gets into the oven, it’s hot, it blows dryness and the air is very dry.

To avoid these sharp fluctuations, metallurgists decided to moisten the blast, ie add a constant amount of moisture to it in the form of steam (up to 25-30 grams per cubic meter of blast). It turned out that in this case, in the furnace under the influence of high temperatures, the water that enters it decomposes and not only stabilizes the work of the furnace: an additional reducing agent is formed in it, hydrogen, which accelerates the reduction reactions. So, the blast furnace inhales the hot, moistened air, but what does this giant exhale? It turns out that valuable fuel is blast furnace gas. In a day, a 5,000-cubic-meter blast furnace produces 27,000 tons of blast furnace gas (it is so called because it leaves the upper part of the furnace – the top). However, it is polluted, it has a lot of ore and coke dust. Therefore, from here he is sent through the pipes to the cleaning in the towers-gas scrubbers, located next to the stove.

To clear the gas from dust, various types of cleaning are used: dry, wet, and electrostatic. In dry cleaning, dust particles precipitate under the action of gravity forces when the speed and direction of the gas flow change. In wet dust collectors, the gas passing through them is irrigated with a liquid (most often water), which is sprayed with irrigation devices. The dust particles suspended in the gas are wetted, become heavier and drop out of the moving gas stream under the action of gravity or inertia. They are collected at the bottom of the gas scrubber or on partitions in it – nozzles of various shapes. The gas can be purified by acting on it with an electric field of high voltage. In this case, inside the gas scrubber, in one way or another, electrodes are placed, to which an electric current is supplied. Gas passing through the gas scrubber is ionized, dust particles receive a certain electric charge and are attracted to the electrodes.

Dust-free gas removed from the dust collectors is used as fuel right there at the plant, even in the blast furnace shop (its calorific value is 850-950 kilocalories per cubic meter). They are heated with air heaters, open hearth furnaces, heating pits in which the ingots are heated before rolling. And the dust accumulating in gas scrubbers through special gates in their bottom is unloaded into railroad cars and sent to agglomeration or pelletizing and again used in a blast furnace.