Obtaining steel in electric furnaces
Currently, about 30% of the world's steel smelting falls on electric furnaces. They have an advantage over other melting units, since they can produce a high temperature of the metal, create an oxidizing, reducing, neutral atmosphere and vacuum, which makes it possible to melt steel of any composition, deoxidize the metal with the formation of a minimum of nonmetallic inclusions - deoxidation products. Therefore, furnaces are used for smelting structural, highly alloyed, tool, special alloys and steels. There are several groups of furnaces: resistance, arc, induction, plasma and electron-beam.
They operate on a three-phase alternating current and have three cylindrical electrodes 9 from the graphitized mass (Figure 4.6).
Fig. 4.6. Arc furnace schematic
The electric current from the transformer through the cables 7 is fed to the electrodes 8, and through them to the electrodes 9 and the metal bath. An electric arc arises between the electrode and the metal charge 3 , the electric power is converted into heat, which is transferred to the metal and slag by radiation. The length of the arc is regulated automatically, the electrodes increase as the combustion proceeds, screwing up with new electrodes. Steel under the furnace is lined with refractory bricks 1 - basic or acidic. The smelting space is limited by walls 5, lined with refractory bricks 4, with a bottom 12 and a vault 6 of refractory bricks. Podinu stuffed with refractory mass, the arch of the furnace is removable. The melting stroke is controlled through the window 10. The finished steel is discharged through the outlet through the chute 2 into the ladle. The furnace has a drive 11 for tilting towards the working window or gutter.
In the main arc furnace, two types of smelting can be performed: on a charge of alloyed waste (by remelting) and on a carbonaceous charge (with oxidation of impurities). In the first case, melting is carried out without oxidation of impurities. The alloying elements are introduced into the steel in the form of ferroalloys. The order of input is determined by the affinity of the alloying elements for oxygen. Melting on a carbonaceous charge with complete oxidation of impurities is carried out if the charge materials used contain phosphorus and significantly differ in the composition of the other elements from the specified grade of steel.
The capacity of arc furnaces ranges from 0.5 to 400 tons. In electric melting, 600-1000 kWh of electricity are consumed per 1 ton of steel ingots. The operating voltage during the melting is 100-200 V in small furnaces and 400-600 V in large furnaces, and the current is measured in thousands and tens of thousands of amperes.
In the induction furnace (Figure 4.7), the metallic charge 1 is melted in a crucible 5 located inside the inductor 4, which is a spiral with many turns of conductive material.
Fig. 4.7. Induction crucible melting furnace:
and - the scheme; b is the appearance; 1 - metal charge - the resulting pure metal; 2 - removable arch; 3 - the drain sock; 4 - the inductor; 5 - crucible
Alternating current is passed through the inductor. For the supply of small-capacity furnaces, lamp and spark generators with a frequency of up to 106 Hz are used, and for large-capacity furnaces, machine generators (up to 104 Hz). The variable magnetic flux created inside the inductor induces eddy currents in the metal, which provide a hundred heating and melting. The crucible 5 is made from acid (quartzite) or basic (magnesite powder) refractories. Induction furnaces with basic lining, high-quality alloyed steels with a high content of manganese, nickel, titanium, aluminum, with acid lining, constructional alloyed steels with other elements are melted. To reduce heat loss, the furnace has a removable arch 2.
An important feature of induction furnaces is the intensive circulation of liquid metal, caused by the interaction of electromagnetic fields excited, on the one hand, by currents passing through the inductor, and on the other - by eddy currents in the metal. Electromagnetic forces exert a static and dynamic effect on the liquid metal, as a result of which its upper part is pressed away from the walls of the crucible, and electrodynamic circulation occurs throughout the volume. The nature of the circulation flows of liquid metal is shown in Fig. 4.7 arrows.
A convex meniscus makes it difficult to treat the metal with slag that flows down to the walls of the crucible; a sufficiently high rate of turbulent metal movement increases the wear of the liner.
Induction furnaces have an advantage over arc: they do not have an electric arc, which makes it possible to smelting steel with a low content of carbon, gases and small element burns. They smelted steel and alloys from alloyed waste by remelting or from pure iron and scrap with the addition of ferroalloys by fusion. The capacity of these furnaces can vary within wide limits: 0.01-25 tons.
Melting of steel in plasma arc furnaces is used to produce high-quality steels and alloys. The source of heat is a low-temperature plasma (3000 ° C) obtained with plasmatrons. The submersible submersible pumps are similar in construction to conventional electric arc furnaces. In them, cathodes of a DC arc discharge are cathodes of plasmotrons, and the anode is a metal to be processed. The arc is blown by a stream of inert gas (usually argon or helium). The charge is rapidly melted, in the neutral gaseous medium degassing of the melted metal occurs, the volatile elements entering into its composition do not evaporate. Plasma high-frequency furnaces (PVP) are usually used for the growth of single crystals and the processing of pure substances.
Electron-beam furnaces (installations)
The transformation of electrical energy into thermal energy takes place directly in the molten metal as a result of the collision of electrons emitted from the electron gun. The melted metal will be fed into the furnace in the form of a consumable electrode, ingot, powder, etc. When it melts, it drips off into the water-cooled crystallizer (mold) or crucible. Melting, which takes place at high temperature and high vacuum, is used to produce extremely pure refractory metals and alloys, ingots of steel and alloys for parts of particularly critical use, etc.
Next, the methods of remelting in electroslag, plasma-arc and electron-beam furnaces are considered. In aggregates of this type, a relatively small amount of special-purpose metal is produced. Perhaps in the future they will find a wider application. While the bulk of the electric steel produced in the world is melted in arc furnaces, and iodine by the word electrostatic it is exactly steel smelted in an arc furnace.
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