At present there are so many types of refractory systems available in the market for tundish lining or tundish coating, as they are often called. However, all of them can be broadly categorized into the following types:
Making a final choice of coating or tundish lining refractories for a plant cannot be made on the basis of mere advantages and disadvantages. The decision of which product should be used depends on the steel mill requirements, knowledge about the process of each plant and the operational exigencies. Therefore, it becomes all the more important to list out the various factors and plant parameters to be examined before taking the final decision such as -
Other factors such as the availability of facilities like preheat arrangements etc. are there which can tilt the balance in a ‘close decision’ situation. Dry tundish coating products are particularly suitable for modern mini-mills where there is normally a limited tundish fleet and hence, need for faster turnaround, and tundishes often being small which makes it easy and quick to install a dry lining.
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Tundish Refractories - Background
Tundish is one of the most important areas of Refractory Application and also one of the biggest single ‘cost center’ in the continuous casting process. This equipment is responsible for receiving the liquid steel from the ladle, subdividing into many strands and controlling the casting speed through flow control devices. Its importance has increased over time and nowadays it is considered a metallurgical reactor, in which a part of the refining of liquid steel can be carried out with safety and quality. The tundish coating or refractory lining is responsible for keeping steel casting temperature and protecting the safety lining from premature wear while it avoids steel contamination by non-metallic inclusions. The primary functions of tundish coating are to protect the high alumina permanent lining and minimize the contamination of the molten steel required for clean steel production. To produce clean steel with good separation of nonmetallic inclusions to the covering slag in the tundish is all the more important to reduce clogging tendency at the SEN (Subentry Nozzle) inlet.
Tundish Coating and Lining Refractories – Types and Assessment
The refractory lining design and quality of refractories used have got great effect on the operational parameters like super heat requirements, longer sequence, speed of the machine etc. The phenomenon like initial cold running-stopper, nozzle choking (tundish nozzle clogging), tundish through etc. are considered to be very important factors for smooth running of CCM.
Tundish lining refractories are expected to take care of the following in continuous casting process:
Switch-over from Refractory Bricks to Castable as back-up lining
This has reference to on of the major integrated steel plants in India having two twin strand casters with a ladle capacity of around 300 Ton and tundish working capacity of 50 Ton along with overflow level of 55 Ton. The tundish is of boat shaped design where liquid steels falls at its middle portion and exit out through two nozzles placed at extreme end. Cold board practice was observed and only tundish nozzle and SEN were heated. Other features included: Alumina bricks (high grog 39% Alumina) as permanent lining, drying of tundish, Silica board (Garnex) as working lining with Quartzite sand in between brick lining and boards, two high grog (39% Alumina) Dams at the bottom, two Weirs (LC 70% Alumina) inserted in the brick lining from the top etc.
The problem they faced was deskulling of the tundishes due to embedded weirs inside the brick lining. Tundish skull was not coming without removing the bricks near the weirs. Some steps were taken. The problem of falling off lining bricks along with the skull still persisted leading to higher cycle time of each tundish and huge muck generation during deskulling. Every time the tundish had to be freshly relined with new bricks and then dried for 4-5 hours before board fixing and sand filling leading to rise refractory cost as well. Then they decided to switch over from brick to castable (LC 70) as permanent back-up lining. With this development the falling-off of permanent lining during deskulling was totally eliminated helping less muck generation and reduced cycle time for each tundish.
Switch-over from Silica Board to MgO Board
Another problem faced by the above steel plant was the Si pick up from silica board lining in tundish which was in the range of about 0.005 - 0.10%. The bulk production of this steel plant is Al killed steel. So, due to this Si pick-up the grades were becoming off chemistry. Silica board had also other related problems: (i) while casting grades having Mn content more than 0.30%, the erosion of boards was high resulting in fusion of sand behind it, (ii) due to dissociation of silica from board, alumina clogging was taking place leading to choking (clogging) of tundish nozzles. Ultimately they decided to replace the silica boards with basic quality i.e. MgO boards.
Initially, after the continuous casting technology came into practice it was then realized the need of some type of disposable lining or coating material to reduce the costs of refractory lining which could also improve thermal insulation of a tundish. Bricks and conventional gunning products were not so reliable. Moreover, they provided no thermal insulation which was gaining importance due to rising fuel costs. The introduction of low density tundish boards offered good thermal insulation and low refractory consumption. However, the main disadvantage of tundish boards was manpower and time required to prepare a tundish lining. A major breakthrough was the development of sprayed linings in the late 1980’s. This material has similar characteristics, such as tundish boards, but the advantage of quick installation with reduced labor. Dry vibe linings were developed at the same time as wet spray materials, however, the high price of phenolic resin powder and associated health and safety issues meant that dry vibe linings were only used in a handful of steel plants. More recently resin free binder systems have eliminated any health and safety issues and in addition this second generation glucose binder based dry tundish coating (and tundish lining) products offer several advantages over other systems. Glucose is cheap and readily available: there are no issues with toxic fumes and since glucose is thermoplastic (as opposed to thermosetting in the case of phenolic resin) as well as it is possible to carry out installation with permanent lining temperatures above 300°C. One can also realize the additional cost savings by extending the life of the tundish permanent lining with some improvements of steel quality and the possibility of increasing sequence life can also be considered.
Despite the concerns about hydrogen pick up and excessive consumption of gas, spray mixes has been a dominant technology for tundish coating. Almost 80% of steel plants in South America use this kind of refractory lining. Although these new technologies present advantages over the spray mixes and are suitable for some specific operational conditions, spray mixes still have a strong presence in the market mainly due to benefits such as flexibility, easy and fast application, low cost, low consumption and high thermal insulation.
In the present article we shall discuss the factors that need to be considered before making a final choice of tundish lining for a steel plant with specific references to some Indian Steel Plants. There are many instances of steel plants switching over from one tundish lining system to other depending upon the perceived and actual benefits obtained. But unfortunately, such experiences, which could be of paramount help for others, are rarely well-documented or published. So, here an effort has also been given to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories (paraphrased below as ‘Case Studies’).
Oxygen picking from to Magnesite (MgO) based basic refractories
It is well known that quality of steel that comes out of tundish is not same as that poured from ladle. Oxygen picking by metal from slag, atmosphere and refractories, particularly in Tundish is a common experience. Slag and tundish lining refractory compositions is one of the few parameters for controlling the tundish nozzle clogging problems. Actually these must be designed to have low enough oxygen potential to absorb inclusions, while not being so reactive that steel composition is altered. In a recently held seminar in India, a major steel producing organization of abroad presented their one such experience and analysis; a brief of which is being paraphrased in the following lines.
The oxygen picked up by metal leads to the formation of inclusions, some of which end up in metal and become detrimental for its quality. Inclusions are generated in the tundish by three mechanisms: re-oxidation, slag entrapment and refractory lining erosion. Inclusions generated by refractory erosion are, in general, larger than those by the other two mechanisms. MgO based refractories used in tundish contain Olivine (Dunite grains) which is a solid solution of Fayalite (Fe2SiO4) and Forsterite (Mg2SiO4). These can react with de-oxidized steel according to following reactions:
Fe2SiO4 = 2Fe + 2O + SiO2
Mg2SiO4 + SiO2 = 2(MgSiO3)
2Al + 3O = Al2O3
MgO + Al2O3 = MgAl2O4
From the study (TEM, SEM analyses) of used tundish refractory they found that the fayalite component of olivine was less. Besides they observed MgSiO3 and MgAl2O4 and concluded that all the above mentioned reactions take place in the tundish