Ladle is a metallurgical vessel that is used to transport and pour out molten metals. They also serve as a vessel that is used for reheating, ferroalloy addition, refining, degassing, and inclusion modification after tapping of the steel. Ladles can have a capacity from 20 kilograms to 300 tonnes. Many non-ferrous foundries also use ceramic crucibles for transporting and pouring molten metal which is also known as ladles.
The ladles are un-geared and pouring from the vessel is brought about with a help of a two-winch crane, where the main winch carries the ladle while the second winch engages a lug at the bottom of the ladle. Engaging the main hoist/winch helps in lifting and lowering of the ladles whereas raising the second winch rotates the ladle on its trunnions which are used for metal/slag dumping.
Steel ladles have porous plugs inserted on the base, which is used for inert gas purging. The carrying out of metallurgical reactions in the ladle is used to adjust the final chemical composition and temperature of the steel after tapping. The simplest mode of ladle treatment is often achieved before or at the tapping where the mixing effect of the stream is utilized to add deoxidizers, slag formers, and alloying agents.
Ladle metallurgy is important from the metallurgical point of view as it helps in the homogenization of chemical composition and temperature of liquid steel, deoxidization, reheating for achieving better superheat and eliminating low temperature chocking in caster, ferroalloy additions, degassing of hydrogen and nitrogen, desulphurization, inclusion modification, improving cleanliness and improving mechanical properties.
Temperature control
The loss of heat takes place by means of radiation from the top surface (although ladle covering compound are being added to prevent radiative heat loss from the molten metal top surface), and conduction through the lining and shell. Temperature drops can range from 0.5° to 2° C per minute even when the ladle is held idle without any additions or treatment. Smaller ladles are more prone to temperature loss due to their high surface-to-volume ratio. The temperature loss is exacerbated in case of improper ladle preheating or ladle put into service after relining (first life ladles), higher ladle circulation time, presence of skull.
Tapping at the right temperature is necessary in order to meet critical temperature windows for teeming or casting operations. Heat losses also depend on other factors like configuration of the tap stream, the thicknesses of the ladle lining and slag layer, holding duration, stirring conditions, and the thermal effects of alloying additions. Temperature rise is achieved in ladle furnace (LF) with the help of graphite electrodes which generates an arc by the use of 8- to 33-megavolt-ampere transformer. Recent LFs have the potential to raise the temperature of the steel from 4° C – 6° C per minute.
The heating efficiency, η , of arc heating is given by:
Where, ∆Tact = the actual temperature increase of the bath, °C, ∆Tth = the theoretical temperature increase of the bath for 100% thermal efficiency, °C, E = the energy consumption, kWh/tonne. The heat capacity of liquid steel is 0.22 kWh/tonne °C; i.e., for 1 tonne of liquid steel, ∆Tth = E/0.22. The heating efficiency increases with an increase in bath weight.
To minimize refractory consumption, heating times in ladle furnaces are kept as low as possible, typically around 15 min. Further measures to shorten the reheating time and thus minimize refractory erosion are the use of a large-capacity transformer, e.g., 35–40 MW for a 200to 250-tonne heat, submerged arcing in the slag layer, argon stirring through a bottom porous plug at a flowrate of approximately 0.5 Nm3/min and a slag layer thickness of approximately 1.3 times the length of the arc.
Stirringstirring
Stirring is brought about by the help of inert gas (argon) infiltrating through the porous plug installed on the bottom of the ladles or by the help of an electromagnetic coil.
The provision of rinsing through a top lance mechanism is usually made to take care of the rinsing at the time of blockage in the bottom plug. Inert gas purging is beneficial in many ways like homogenization of the temperature and composition, floatation of alumina inclusions, facilitates desulphurization, dephosphorization and slag killing.
The typical gas bubbling rates are (eg-mild purging-6.89 bar or 5N cum/hr per plug, moderate-8.67bar or 10 N cum/hr per plug, vigorous-12 bar or 20 N cum/hr per plug).
Injections
Materials, such as calcium-silicon, zirconium, and rare-earth metals, are often enclosed in thin steel tubes and are fed into the steel. Powdered metal is fluidized by argon in a pressure vessel and injected by a refractory-lined lance deep into the liquid steel. It reacts quickly with the steel owing to the high surface area. Deep injection is beneficial when adding materials such as calcium or magnesium, which evaporate at steelmaking temperature, and ferrostatic pressure prevents these metals from escaping by evaporation.
Deoxidation:
Deoxidation of the steel can be brought about with ferromanganese, ferrosilicon, silicomanganese and aluminium. The use of synthetic slags in the ladle is an integral part of ladle metallurgy because of the requirements necessary to produce ultraclean steels and increasing demand for extra low sulphur contents. The steel is first deoxidized partially with Fe/Mn and/or Fe/Si, followed by a final deoxidation with aluminium. Such a practice helps in minimization of nitrogen pickup during tapping, minimization of phosphorus reversion from the carried-over furnace slag and minimization of aluminium losses due to reaction with carried-over furnace slag.
Desulphurization
Many powder-injection stations are used to facilitate the desulfurization process. Calcium-silicon alloy used as a desulphuriser, contains 30 percent calcium. Calcium reacts with sulphur to form a very stable compound calcium sulphide (CaS). Injecting four kilograms of calcium-silicon per ton of steel can significantly reduce sulphur content for example, from 0.016 to 0.004 percent
For steel grades where there is a restriction to silicon additions, a magnesium-lime mixture is used. Magnesium is a good desulfurizer and a good deoxidizer as well. Lime plays a dual role like Magnesium because it helps to prevent the very low-melting magnesium powder from melting inside the lance.
Very low sulphur content is required, i.e., 20 ppm or less especially for line pipe application. These low sulphur contents can be achieved only by steel desulfurization in the ladle in the presence of a calcium aluminate slag when the steel is fully killed. For the required degree of desulphurization to take place within a practical time span, good mixing of steel and slag is essential. The rate, at which the sulphur can be removed, is strongly recommended by the gas flow rate during the rinsing of steel. Another method for achieving very low sulfur content is by the injection of fluxes into the ladle. A typical flux used for desulphurization contains 70 % CaO and 30 % CaF2.
De-phosphorization:
Ladle dephosphorization may be necessary for BOF shops in which hot metal with a high phosphorus content is charged and where there is no capability of dephosphorizing the hot metal prior to charging to the BOF. Removal of phosphorus from the steel in the ladle is achieved by treating the steel with lime-based oxidizing slags containing iron oxide.
Inclusions Modifications
Calcium treatment of liquid steel is normally employed to change the morphology of the inclusions. As a result of the treatment with calcium, the alumina and silica inclusions are converted to liquid calcium aluminates or calcium silicates. These liquid inclusions are globular in shape because of surface tension effects. This change in inclusion composition and shape is referred to as inclusion modification or morphology control. Since the boiling point of calcium is 1491-degree C, calcium is a vapor at the steel-making temperature. Hence measures are to be taken to ensure its proper recovery in the steel bath.
In calcium-treated low-sulphur steels, the grain boundary precipitation of MnS during solidification is suppressed as a result of the precipitation of sulphur as a Ca(Mn)S complex on the calcium aluminate inclusions.
Degassing
During the process of steelmaking gases such as oxygen, hydrogen and nitrogen get dissolve in steel. Degassing is required to remove or lower the concentration of nitrogen and hydrogen in steel. Initially, vacuum degassing was used primarily for hydrogen removal but later used for the production of ultra low-carbon (ULC) steels with carbon contents of 30 ppm or less. Furthermore, a relatively new family of steel grades, the so-called interstitial-free (IF) steels with carbon and nitrogen contents of 30 ppm or less, has appeared on the scene. To achieve these low carbon and nitrogen contents, a treatment under vacuum is mandatory.
Degassing can be carried out either by placing ladle containing molten steel under vacuum (non-recirculating system) or by recirculation of molten steel in vacuum (recirculating system). Examples of recirculating systems are RH, RH-OB, RH-KTB, and DH etc. processes and examples of non-recirculating systems are ladle or tank degassers, including VAD (vacuum arc degassing) and VOD (vacuum oxygen decarburization), and stream degassers.
One of the motive behind treating of steel in an RH or RH-OB (KTB) degasser is to lower the dissolved oxygen concentration of the steel by means of carbon deoxidation before adding aluminium to kill the steel completely.
Some nitrogen removal from liquid steel during vacuum degassing is possible provided the steel is fully killed (low oxygen level) and has low sulphur content.(The oxygen and sulphur are surface-active elements which hinder lowering of nitrogen levels to considerably lower limits).
Decarburiser
The liquid steel is decarburized and refined in the AOD vessel to less than 0.05% carbon. The key feature in the AOD vessel is that oxygen for decarburization is mixed with argon or nitrogen inert gases and injected through submerged tuyers. This argon dilution is effective in reducing unwanted oxidation of precious elements present in specialty steels, such as chromium.
AOD is widely used for the production of stainless steel and specialty alloys such as silicon steels, tool steels, nickel-base alloys, and cobalt-base alloys. The process is popular because of high yield and low material cost. Other benefits include accuracy in chemistry control down to 0.01 % carbon and lower, rapid desulfurization to less than 0.001 %, and lead removal to less than 0.001 % which results in cleaner steel and improved productivity.
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