Tundish - An Overview
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. When more than one heat is cast in sequence, the time between the completion of the first ladle and opening of the second ladle is taken care of by the metal in tundish. We continue from where we left off at Trends and Developments in Continuous Casting Tundish Lining Refractories [Read]. In this post 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 in India and abroad 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’).
Fig: Ladle-Tundish arrangement in Continuous Casting
Tundish Lining Refractories - A Comparative Evaluation
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 through etc. are considered to be very important factors for smooth running of CCM.
Normally tundish lining refractories are expected to satisfy the following in continuous casting process:
- Low temperature losses even in long sequential castings.
- Low turn around time.
- Long campaign life and less maintenance time to reduce the cost of refractories.
- Simple and reliable heating system.
- Less labour requirement.
- Easy deskulling and less debris generation.
- Better lining integrity required for steel cleanliness.
- Simple, quick and environment-friendly installation.
- Low specific consumption per ton of steel and specific cost per ton of steel.
At present there are different types of refractory systems available in the market for tundish lining. These can be broadly categorized into 5 major types:
- Bricked lining
- Dry (in-situ formed)
As can be deduced from the analysis done in our previous post [Trends and Developments in Continuous Casting Tundish Lining Refractories], it is not easy to make a final choice of tundish lining for a plant only on the basis of the advantages and disadvantages of the above refractory systems. It may also be that there is no final choice for a particular plant - and they may be well advised to use at least two of the optimum types of lining. However, operational exigencies usually demand the choice of a single type of lining, and therefore, it becomes all the more important to list out the various factors and plant parameters to be examined to make the final decision:
- Tundish size and geometry needs to be looked at closely. Installation problems increase with the increase of tundish size for boards since it would require many labours at a time and therefore, significantly affect the tundish availability. However, for small tundish, boards are quick and easy to install while the wet system can be quite complicated and expensive. With sprayables, while installation is quick, the drying time is long. Dry systems usually offer the best turnaround for a large tundish. For any tundish of difficult shape (geometry), it is preferable not to go for dry linings since formers can be very complicated and expensive.
- The extent of sequencing has also an impact on the final selection of refractories. Boards usually lose out when high sequences are practiced, due to joint penetration problems. Dry linings can also be a handicap for long sequencing plants, since it can lead to a very high over-consumption as the wear lining thickness increases to accommodate the reducing thickness of the permanent lining.
- For any plant which finds it convenient to adopt ‘cold start’ practice for at least some of its steel grades, the board and dry systems become ideal. Spray may also be used but extensive curing can impose cost as well as time penalties.
- Gunning is not suitable for ‘cold start’ practice. Similarly silica boards usually do not lend themselves to a hot tundish practice.
- For tonnage steel grades, where steel quality is not so critical, silica board linings can be a very attractive low cost lining system. However, if steel quality is of prime importance spray and dry systems are the best, in fact even better than hot magnesite boards. Gunning can lead to micro-inclusions in case of over-spray.
- If capital is not a constraint and sophisticated equipment can be procured then both spray and dry systems are ideal. Especially spray linings are amenable high degree of automation and use of robotics, leading to cost reductions significantly. However plants facing fund crunch, board system may be the best because of no equipment expenditure at all. Plants with small capacity tundishes need to think twice before spending money on fancy robotics spray equipments, since economics may be questionable.
- The number of tundishes available is an important determining factor. Plants which have to work with a small inventory of tundishes will find boards very expedient because of their excellent deskulling performance heat after heat. If acid linings are acceptable, silica boards are the cheapest route available, whatever be the other conditions.
- Tundish size is a major factor in determining the tundish lining costs, with costs increasing significantly per ton of steel as tundish size decreases (especially true for the magnesitic system). Sequencing is also extremely important in reducing costs. With larger ladle sizes and large degree of average sequencing, the larger tonnage steel plantshave a significant advantage in maintaining a low tundish lining cost, even with the use of magnesite-based basic lining systems.
- Other factors such as the availability of facilities like preheat arrangements etc. are there which can tilt the balance in a ‘close decision’ situation.
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 of tundish nozzles.
Ultimately they decided to replace the silica boards with basic quality i.e. MgO boards.
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. 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 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.