Steel is usually made from pig iron by bringing, about the following changes.
1) Reduction of carbon from 4.5% in pig iron to 1.5% in steel.
2) Removal of the impurities such as Si, Mn, P etc. present in pig iron through slag formation.
3) Addition of the alloying elements such as Cr, Ni. V. Mo and W to give the steel its desired properties.
Important methods for the manufacture of steel are.
(1) BESSEMER PROCESS
This process involves the oxidation of impurities of pig iron by oxygen of the air into volatile gases of slag formation. It is followed by the addition of required amount of carbon and other alloying elements. The process is carried out in steal shaped furnace called Bessemer converter.The furnace is made of steel plates and is internally lined with silica (SiO2) /or Magnesia (MgO) bricks depending upon the nature of impurities present in pig iron.
When the impurities present are Si, P or S. a ,basic lining of magnesia (MgO), used the process is called basic Bessemer process. On the other hand. if the impurities present are Si or Mn, a lining of silica (SiO3) bricks is used, the process is called acid Bessemer process. The Bessemer converter can be tilted in any position and has a number of holes at the bottom to admit the blast of hot air. The molten pig iron is added into the Bessemer converter. A blast of hot air is allowed to enter through the holes. As the air passes through the molten metal, it oxidizes the impurities.
After the removal of the impurities, carbon present in the pig iron is oxidized to carbon monoxide which bums with a blue flame at the mouth of the converter. When carbon monoxide flame subsides, a calculated amount of spiegeleisn (an alloy of iron, manganese and carbon) is added. The blast of air is turned on again for a few minutes to homogenize the mass. At the end, the converter is tilted to pour out the molten mass.
(2) OPEN HEARTH PROCESS
This is the most modem process for the manufacture of steel and is based on the regenerative system of heat economy. The charge consisting of a mixture of pig iron, scrap iron and hematite ore free from carbon is taken into the trough or hearth of the furnace. The heath of the furnace is lined either with dolomite or magnesia or silica depending upon the nature of the impurities present. The ehargc is heated by means of the flame obtained by burning fuel gases. The hot ,waste gases from the furnace are allowed to escape through the fire-bricks check-work “C” and “D” who absorb the heat of the escaping gases.
A small quantity of the charge is drawn out from time to time to analyses the carbon content. The carbon content of steel is adjusted by adding the appropriate amount of spiegeleisn. The steel obtained by open-hearth process is used for making rail wheels, axles, springs, guns, girders and ships.