► Modern Processes of Steel making
Modern steel making process uses a blast furnace for manufacturing steel. It can be divided into two categories: primary and secondary. Primary steel making includes “converting liquid iron from a blast furnace and steel scrap into steel via basic oxygen steel making or melting scrap steel and/or direct reduced iron (DRI) in an electric arc furnace”.
The secondary process involves alloying. Here alloying agents are like manganese, chromium, nickel, and vanadium are added and dissolved gasses are sent through the lower portion. Dissolved gasses play an important role in controlling the quality of the steel.
► Primary Steel making Process
It is the process of making steel from carbon-rich molten pig iron. As oxygen is passed through the molten pig iron to lower the carbon content of the alloy, the process is also called basic oxygen steelmaking. The word basic is added with it for the chemical nature of the refractories- magnesium oxide and calcium oxide. The basic principal of it is to lower carbon content in pig iron in order to make pure steel from it.
In this method, iron ore is mixed with coke and heated highly in order to form an iron-rich clinker called ‘sinter’. Sintering is an important part of the overall process. It plays a significant role in reducing waste and provides an efficient raw material for iron making. The process emphasizes on continuous casting. Exothermic heat is created with the continuous oxidation reactions at the time of blowing.
● Molten pig iron is poured from a blast furnace into a large refractory-lined container called a ladle.
● In the next step, the metal in the container is sent directly for applying basic oxygen steelmaking process. After that high purity oxygen at a pressure of 100-150 psi (pounds per square inch) is sent into the vessel at the supersonic speed that directly hits the surface of the iron bath through a water-cooled lance kept at few feet above the bath. The purpose of doing this is to reduce phosphorus, sulphur and silicon phosphorus before charging the hot metal into the converter. In the case of external desulphurising pretreatment, a lance is lowered into the molten iron in the ladle and quite a few hundred kilograms of powdered magnesium are added. Consequently, the sulphur impurities get reduced and become magnesium sulphide in a violent exothermic reaction.
● After that the sulfide is raked off. The decision to pretreat depends on the quality steel needed. The BOS process is autogenous. It means the thermal energy required for this process is produced during the time of oxidation process. Therefore, maintaining the right charge balance is highly necessary. It fully controls the hot metal, melt, cold scrap, etc. The BOS vessel can be tilted up to 360° and it can be inclined towards the deslagging side for charging scrap for giving the hot metal required the charge. For charging the BOS vessel iron scrap (25%-30%) or steel is used.
● Now this is the next step to set vessel upright and cooling it with cold water. After that a copper tipped lance with 3-7 nozzles is gradually lowered down into it and pure oxygen is passed through it at supersonic speed. It oxygen must be 99% pure. When it will be passed over the hot metal, the carbon within it will be dissolved to produce steel. At first, the carbon will form monoxide and then carbon dioxide. This carbon dioxide increases the temperature and raises the temperature up to 1700 °C. It melts the scrap, reduces the carbon contents (present in molten iron) and removes unnecessary chemical elements.
► Secondary steel making
Apart from primary steelmaking process, there is secondary steelmaking process commonly performed in ladles. This is this method is often called as ladle (metallurgy). There are a few operations conducted in ladles include de-oxidation (or “killing”). There are alloy addition, inclusion chemistry modification, inclusion removal, homogenisation, vacuum degassing, and de-sulphurisation.