Steels are alloys of iron with carbon, the content of which does not exceed 2.14%. Steel – the most important structural material for engineering, transport, etc. Steelmaking is the production of steel from iron and steel scrap in steelmaking units of metallurgical plants.
Steel production in converters
The converter is a pear-shaped vessel. The upper part is called a visor or helmet. It has a neck through which liquid iron and drain steel and slag. The middle part has a cylindrical shape. In the lower part there is an additional bottom, which is replaced with a new one in the process of wear. An air box is connected to the bottom, which receives compressed air.
The capacity of modern convectors is 60-100 tons and more, and the air blast pressure is 0.3-1.35 meganewtons per meter. The amount of air necessary for processing 1 ton of cast iron is 350 cubic meters.
Before pouring cast iron, the convector is turned to a horizontal position, at which the holes of the tuyeres are above the level of the cast iron. Then it is slowly returned to a vertical position and at the same time an air blast is given, not allowing the metal to penetrate through the apertures of the tuyeres into the air box. In the process of blowing air of liquid iron burned silicon, manganese, carbon and partly iron.
When the required carbon concentration is reached, the convector is returned to the horizontal position and the air supply stops. The finished metal is deoxidized and poured into a ladle.
Steel production in open-hearth furnaces
In open-hearth furnaces, fuel oil or pre-heated gases are burned using hot blasting.
The furnace has a working (smelting) space and two pairs of regenerators (air and gas) for heating air and gas. Most open-hearth furnaces are heated with a mixture of blast furnace, coke oven gas and generator gas. Natural gas is also used. The Marten furnace, which operates on fuel oil, has generators only for heating the air.
Shirted materials (scrap, cast iron, fluxes) are loaded into the furnace filled with the machine through the filling ports. Warming of the charge, melting of metal and slag in the furnace occurs in the melting space when the materials come into contact with the torch of hot gases. The finished metal is discharged from the furnace through openings located in the lowest part of the bottom. For the time of melting, the outlet is clogged with refractory clay.
The process of melting in open-hearth furnaces can be acidic or basic. In the acid process, the refractory brickwork of the furnace is made of the dinas of the brick. The upper parts of the bottom are welded with quartz sand and repaired after each melting. In the smelting process, an acidic slag with a high silica content (42-58%) is obtained.
Steel production in electric furnaces
Two types of electric furnaces are used for steel smelting: arc and induction (high-frequency) furnaces. The first of them have received a wider application in the metallurgical industry.
Arc furnaces have a capacity of 3 – 80 tons or more. At metallurgical plants, furnaces with a capacity of 30 -80 tons are installed. In electric furnaces it is possible to obtain very high temperatures (up to 2000 ° C), to melt the metal with a high concentration of refractory components, to have a basic slag, to clean the metal well of harmful impurities, to create a reducing atmosphere or vacuum (induction furnaces) and achieve high deoxidation and metal degassing.
Heating and melting of the charge is carried out at the expense of heat radiated by three electric arcs. Electric arcs are formed in the smelting space of the furnace between vertically suspended electrodes and a metal charge.
In steelmaking furnaces, coal and graphite electrodes are used. The diameter of the electrodes is determined by the power of the current consumed and is 350 – 550 mm. During the melting process, the lower ends of the electrodes are burned. Therefore, the electrodes are gradually lowered and, when necessary, built up from above.
Technology of steel smelting in arc furnaces
In electric arc furnaces, high-quality carbon or alloy steel. Usually, for smelting steel, the charge is used in a solid state. Solid charge in arc furnaces with basic lining is used for melting of steel with charge oxidation and during metal melting without oxidation of charge.
When melting steel by the method of remelting, the furnace does not load iron ore; There are no conditions for the boiling of the bath. The charge consists of doped waste with low phosphorus content, since it can not be removed to slag. To reduce the carbon content, 10-15% of soft iron is added to the charge. The primary slag formed from the melting of the charge is not removed from the furnace. This preserves the alloying elements (Cr, Ti, V), which pass from the slag to the metal.