#4 - Steel Refining

Desulphurization and dephosphorization are two important processes in steel-making that are used to reduce the levels of sulphur and phosphorus present in the steel. The main goal of these processes is to reduce or remove the sulphur and phosphorus content in order to produce a desired grade of steel with ideal properties such as increased strength, improved weldability, corrosion resistance, and ductility. S & P being impurities, well mainly S, their presence in steel also affects the machinability, i.e., while rolling into longer sections, these impurities develop unwanted air pockets or a.k.a. blow holes within the rolled product.

The most common methods for desulphurization and dephosphorization include vacuum degassing, basic oxygen furnace (BOF) blowing, ladle metallurgy treatments, and electro-slag remelting. Vacuum degassing is a process where molten steel is heated under vacuum conditions which causes sulphur to be released from the melt as hydrogen sulphide gas. BOF blowing is also known as the basic oxygen process (BOP) where elemental oxygen reacts with harmful impurities such as sulphur that are present in molten steel. Ladle metallurgy treatments involve adding fluxes or other forms of reagents such as lime or calcium carbide into the molten steel which helps bind the sulphur for easier removal. Lastly, electro-slag remelting involves melting scrap metal to form ingots using an electric current which helps remove contaminants such as sulphur, by converting them into slag that can then be spooned out from the melt.

It is important to use a combination of these techniques when attempting desulphurization and dephosphorization as each method has its own advantages and disadvantages depending on what type of steel you are working with. For instance, vacuum degassing works well for low-carbon steels but may not be effective for higher-carbon steels whereas ladle metallurgy treatments work well for high-carbon steels but may not be suitable for low-carbon steels due to their increased susceptibility towards oxidation during these treatments. Ultimately combining different methods will ensure an optimal result when it comes to reducing sulphur and phosphorus content in steel while maintaining desired properties such as tensile strength, elongation rate, corrosion resistance etc.

In summary, steel refining is practised across all melting divisions, not only for the benefit of the user but also for the benefit of the producer. Impurities in steel are found in various forms, and we expect such impurities to present because of melting scrap metal in the induction furnaces that generate flux. Such impurities are to be removed, or at least reduced, so as to achieve an efficient rolling process, as well as provide a quality end product of controlled chemistry.

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