Steelmaking processes involving calcium metal aim to remove impurities and improve quality. It is used to deoxidize, desulfurize and increase the fluidity of the steel melt.
It also breaks up interdendritic Al2O3 galaxies into fine spherical Type III inclusions that do not clog continuous caster nozzles.
Ca addition is normally made either prior to or essentially simultaneously with the oxidizable components being added to the bath.
Steel is a versatile, strong and durable metal. It has played a key role in the development of ancient, medieval and modern technological societies. It is made by alloying iron with other elements. Some of these are intentionally added to improve its properties while others are present incidentally.
Once a piece of metal has been worked it often needs to be cleaned. This is done by submerging it in a solution called pickle liquor, which contains acid. The acid eats away any oxide or other impurities that are left on the surface of the metal.
Calcium compounds are widely used. There are vast deposits of limestone (calcium carbonate) which is used directly as a building material and indirectly for cement. When burned in kilns it reacts vigorously with water to produce slaked lime (calcium hydroxide). It is also added to molten steel to remove impurities and enhance its properties. Copper, Nickel, Chromium and Molybdenum are typically found as residuals in finished steel.
The steel ladle is a large vessel that contains molten metal for pouring into a mold in the steel plant. The refractory lined container can either be on wheels or slung from an overhead crane. It is designed to withstand the high temperature and corrosive nature of the steel melt and slag.
The operation of the ladle involves several processes such as deoxidation, desulphurization, dephosphorization, controlled addition of alloying elements and inclusion modification. Adding calcium accomplishes many of these operations, but it also improves physical properties such as toughness and machinability of the resulting cast steel.
The liquid steel in the ladle is constantly losing heat due to exothermic reactions, thermal radiation from the slag and lining surfaces and evaporation of slag from the surface of the metal. The rate of temperature drop is slowened by stirring the slag, gas injection and the application of vacuum. However, the effective effectiveness of degassing decreases from top to bottom of the steel bath because the ferrostatic pressure of the column of liquid steel slows down the emergence of the ascent of the gas plume.
Steel can be refined in a process known as ladle metallurgy (sometimes called secondary steelmaking or ladle refining). This is a key step in the smelting of iron in BOS and EAF furnaces. This is where the addition of calcium can be useful.
During this step, which is also known as slag treatment, solid calcium additives are injected into the liquid steel through a cored wire. This helps to deoxidize, desulphurise and modify inclusions in the liquid steel. This helps to prevent clogging in the casting nozzles and ensures a high castability.
For optimum efficiency, it is crucial to know how far the cored wire travels before its sheath melts and the filling material is released into the liquid steel. For this reason, a mathematical model has been developed to predict the melting time of the sheath and to optimise the operating conditions of the cored wire. This allows for maximum calcium recovery from the slag.
Vacuum degassing is a process used to remove hydrogen, nitrogen, and oxygen from the molten steel. It takes advantage of the phenomenon that carbon molecules, when exposed to a reduced-pressure environment, combine with oxygen to form the water vapor gas carbon monoxide. This gas is then rejected from the liquid steel.
The basic concept is to vacuum the molten steel in the ladle before it is poured into the mold (Fig. 2). There are two basic methods of vacuum degassing: stream degassing and ladle degassing. The effectiveness of both depends on the amount of surface area the molten steel has when it is surrounded by a lower-pressure atmosphere.
Stream degassing is the more common of these techniques. It involves drawing a vacuum on the steel as it is poured from a tank (called a tundish) into the casting mold. During this phase, the steel breaks up into droplets which dramatically increases its exposed surface area.
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