Importance of Tundish in the Continuous Casting Process
Tundish is an intermediate vessel that is intended to deliver the molten metal to the molds at a designed throughput rate and temperature without causing contamination leading to the formation of macro and micro inclusions. A tundish also acts as a reservoir during the ladle change over period and continues to supply steel melt to the mold. A tundish has the capability to reduce some fraction of macro and micro inclusions from the melt. The latest development in tundish operations has given steel makers the scope to adjust chemical compositions and melt temperature to an appropriate level for feeding into the mold.
A tundish is a refractory-lined vessel and has a refractory-lined lid on the top. The lid is mainly made up of iron frame and is protected by the help of 70% low cement castable. The tundish bottom has one or more nozzle port with slide gate or stopper rod for controlling the metal flow. The vessel is often divided into two sections an inlet section, which generally has a pouring box and where steel melt is fed from the ladle; and an outlet section from which melt is fed into the mold.
Tundish furniture
Various flow control devices, such as dams, weirs, baffles with holes, and inert gas purging beam, etc., arranged along the length of the tundish is some of the conventional furniture. They are manufactured from 70% alumina refractories but sometimes magnesite based refractories are used when there is an erosive flow of metal. They are used to improve the cleanliness of the molten steel, prevent short-circuiting, impart thermal and chemical homogeneity and also reduces the chilling effect during the start of casting. Dams work as barriers, which are placed at the tundish bottom. When steel flow across the dams, it provides an upward momentum to the flowing liquid and hence inclusions are forced to float upward and get absorbed in the tundish flux. Weirs are refractory plates which are fixed across the tundish and a gap is left at the tundish bottom which forces the fluid to flow downward. The primary function of the weir is to prevent ladle slag carried over from reaching the tundish nozzles. Baffles with holes is one of important refractory which can be installed all over the tundish, from top to bottom, and contain a series of angled holes to produce a directional flow of the steel, which helps to prevent slag-metal mixing during ladle exchanges when the tundish level drops. An additional type of furniture is the impact pad. It is inserted on the tundish bottom below the ladle shroud or sometimes below the turbulence inhibitor box and is used to reduce the impact stream energy, optimize the flow characteristics, and protect against steel penetration into the permanent lining. Turbulence inhibitor is a special designed refractory flow control device which functions to eliminate initial splashing, dissipate the kinetic energy from the incoming jet, reduce turbulence in the entire vessel, and optimize the flow for enhanced particle removal and temperature distribution in the strand.
Advances made in flow modifying devices involve increasing the surface area on the shape to improve inclusion capture rate. This can be done by rough surface finish or intricate designs like dimpled surfaces.
Latest tundish technology for clean steel production.
Various technologies such as a long nozzle (refractory shroud) or an inert gas shrouding pipe have been implemented to prevent air reoxidation and slag emulsification. Similarly, melt flow control devices which are still under exploration, has the potential to enhance flotation of inclusions formed and also change flow pattern in the pouring zone thereby reducing turbulence and surface-level fluctuations during the process. The implementation of active control of the melt temperature in a tundish has also contributed to interrupted casting. These measures have proved to be quite successful, at least during the steady-state tundish operation. Longer path is preferred to prolong melt residence time to promote the flotation of macro inclusions. The melt delivery rate into the mold is held constant by keeping the melt depth in the tundish at a constant level. Any additional delivery rate control is exerted by the slide gates or stopper rods placed at the exit ports of the outlet compartment.
There are many parameters that have to be optimized during the tundish operation to achieve the best results, including the following key points:
· Increasing residence time.
· Preventing short circuits.
· Minimizing dead volumes.
· Optimizing flow pattern during ladle exchanges and steady-state.
· Improving temperature distribution and concentration homogenization.
· Promoting inclusion removal.
· Maximizing steel yield.
· Reducing clogging and reoxidation.
· Ensure inclusion assimilation by a captive and noncorrosive slag.
Scope for improvement
· Better steel teeming ladle shrouding.
· Better argon shielding.
· Better tundish wear lining.
· Improvement in tundish furniture used.
· Proper tundish stirring.
· Using computational fluid dynamics technology for the removal of inclusion.
· Thermochemical simulation technology for inclusion formation.
