The life of a shroud plays an important role as far as profit margins for the refractory makers are concerned. Apart from that, premature failure of the shroud leads to open type unshrouded teeming of metal from the ladle into the tundish. Unshrouded casting causes a large amount of air entrapment and bath metal turbulence. The bath turbulence leads to the emulsification of slag-producing macro inclusions in the steel. Entrapped air produces alumina inclusions which cause chocking of the nozzle which may lead to the closing of a particular strand or even aborting casting in extreme cases. Higher concentrations of N2 and O2 in steel produce pinholes and blow holes respectively in the final product.
The shroud cleaning with the help of oxygen during ladle changes removes any metal attached to the neck or the body of the shroud and makes it appropriate for further re-use. But during the course of cleaning, a very low viscosity, high-temperature slag/steel is produced which can penetrate deeply into the refractory if there is no carbon present to protect it. Oxygen lancing in the top of the shroud is an area of high wear. Restricting too much lancing and less oxygen pressure is a key to achieving high shroud life. The surface condition of the shroud bowl is also important to prevent air aspiration.
The ramping of the tundish bath level(2-3 tons ) is to be strictly followed in order not to restrict the shroud erosion to a particular position, rather spreading it over a band.
During ladle change, the shroud cools down and when a new heat is tapped through it, the shroud suffers from thermal shock. A vertical crack is a characteristic of thermal shock failure due to the action of the radial expansion forces. Ladle change over time to be kept as low as the possible and free opening of the ladle is to be ensured because assisted open may result in higher temperature loss(resulting in more thermal shock)due to time delay and may even result in crack formation.
Ladle chocking to be inhibited as such condition produces an asymmetric flow of metal and aggravates abrasion and local stresses in the material. Perfect verticality of the shroud should be maintained. Inclination causes a higher impact in the neck area and may result in breaking of the shroud from the neck region and the gasket may also get damaged due to action of concentrated forces. (Concern for steel makers- Inclined shroud causes more air infiltration. Biased jet flow is formed which hits the metal bath and the turbulence is unevenly distributed. Steel-argon plume becomes biased leading to eccentric open eye formation. It can give rise to intense slag-steel emulsification and slag entrapment)
The slag should also be maintained in a fluid condition. Tundish covering compound is to be periodically added in every heat to prevent loss of heat and crust formation of the slag. Slag-free tapping practices and slag detection devices are to be installed to prevent slag carryover. Less volume of the slag in the tundish is to be ensured. Any unwanted jam in the tundish due to unshrouded casting should be immediately cleaned by lancing before dipping the shroud into the bath.
The steel can of the shroud should sit inside the collector nozzle of the ladle slide gate allowing the steel to flow only through the straight long nozzle. If it doesnot fit properly bouncing of steel can take place causing spitting of steel from mouth and air aspirations as well. The impact of the stream and its bouncing effect can wear the surface causing the nozzle to break from its neck.
0.5% Zirconia in the slag line can help achieve higher life as well. Better collar support for the neck region from the shroud bracket is necessary. Shroud should be kept in fairly tight and close proximity to the shroud bracket in order to restrict sideways movement and prevent impact during shroud removal at the end of every heat.
Note: The material quality plays a vital role in determining the shroud life. Special attention is to be given to the composition when higher life is to be achieved.
© Metal world insight
