Showing posts with label Tundish. Show all posts
Showing posts with label Tundish. Show all posts

April 5, 2009

Gunning and Spraying : Fundamental Differences in these two methods of Tundish Wear Lining


There is no dearth of instances of steel plants switching over from one tundish lining system to other depending upon their perceived and actual benefits obtained. But well-documented published data of such experiences, which can be of immense help for others, are either scanty or sparsely available. In our article A Comparative Evaluation of Different Types of Tundish Lining Refractories it has been tried to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories. Here we will try to understand the fundamental differences between Gunning and Spraying, the pros and cons of Gunning vis-à-vis Spraying in tundish wear lining and their root causes.

In case of tundish wear lining whether it has been done by Gunning or Spraying, it is the coating material’s performance and its adjustability with the permanent lining which enhances the life of tundish. The properties of the coating material that determines the performance are -

>> Optimized packing density for good insulating property to reduce the heat loss through refractory.
>> Controlled shrinkage that helps in easy deskulling and avoid crack formation due to high shrinkage.
>> Reduce the slag infiltration to extend the service life of coating material, etc.

Gunned linings in tundish are said to have been commercially started in Japan to overcome some of the problems of bricked linings. Initially these were alumino-silicate based and later converted to basic type magnesite based to assist with metallurgical practice. Conventional tundish gunning materials are designed to have a low strength between 1000 - 1250OC. This feature assists in formation of a weak zone between the backup lining and the sintered zone, which in turn facilitates easy deskulling. One of the many disadvantages of tundish gunning material is the shrinkage at high temperature which deteriorates the performance of gunning material. A high shrinkage causes high stress and subsequent crack formations during operation whereas a low shrinkage can be a barrier for easy deskulling. To know more about advantages and disadvantages of different tundish lining materials and practices see our article Trends and Developments in Continuous Casting Tundish Lining Refractory Practices.

The most commonly faced problems of using a gunning machine or in the process of gunning are -

>> Dust formation during gunning.
>> High rebound losses leading to wastage and high consumption of material.
>> Difficulties in applying variable thickness leading to metal penetration and insufficient
permanent refractory lives, and
>> Difficult deskulling.

Gunning vs Spraying
Whereas when the material is used on a spray gun i.e. through spraying it has the following benefit -

>> No dust formation during application.
>> No rebound loss hence minimal loss of material.
>> The lining thickness was better controllable, thus increasing permanent refractory lives.
>> Deskulling was better.

The root cause of the above can be found in the fundamental differences between Gunning and Spraying as explained in the adjacent figure. Since homogeneous mixing is possible in spraying (before the product is applied), the incorporation of special chemical additives can help to improve thermal stability properties of the lining and also impart good flexibility.


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March 7, 2009

A Comparative Evaluation of Different Types of Tundish Lining Refractories

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’).
Refractory Technology: Continuous Casting Ladle and Tundish image
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:
  1. Bricked lining
  2. Gunnable
  3. Board
  4. Sprayable
  5. 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.

Case Studies

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.

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Trends and Developments in Continuous Casting Tundish Lining Refractory Practices

INTRODUCTION

Continuous casting has been a landmark achievement in the area of steel making. Continuous casting refractories directly control the molten steel in the last stage of liquid steel processing and these are therefore, required to have high stability and special properties. In any continuous casting shop, tundish acts as a buffer vessel between steel ladle and mould. It serves the purpose of reservoir as well as fulfills certain metallurgical functions like floatation of inclusion, control of flow to the moulds, thermal and chemical homogenization etc. Small wonder that over the years, there have been dramatic changes in tundish refractory practices around the world. From a mere reservoir and distribution vessel, the tundish started to be viewed as a steel refining vessel and a totally new field in the process of steel making technology emerged known as Tundish Technology. Here we will outline the progression in the developments of tundish lining along with their main features, advantages and disadvantages and thereby help the individual steel maker decide what is best for his plant.

TYPES OF TUNDISH LINING REFRACTORIES


There is a host of different tundish lining practices which can be categorized into 5 major types (also in a roughly chronological order):
  1. Bricked lining
  2. Gunnable
  3. Board
  4. Sprayable
  5. Dry (in-situ formed)

Refractory Bricks Lining


With the first commercial introduction of continuous casting in around 1960’s initially the same concept of refractory brick lining technology as used in other metal containing vessels was applied to continuous casting tundish lining. These bricked linings were of high alumina type used in direct contact with liquid steel, after intense heating. It was essentially an extension of ladle refractory practices to the tundish.

Advantages:
  • low risk of H picking by molten steel
  • no sand
  • low inventory
  • no investment in equipments
  • low washout risk
Disadvantages:
  • intensive curing required
  • highly labour intensive
  • poor insulation
  • late stage temperature drop in casting operations due to high thermal conductivity of the brick lining resulting into metal heat loss affecting the metallurgical parameters
  • “Cold Start” not possible
  • large tundish fleet required
  • difficult deskulling (stripping)
  • joints
  • long tundish preparation time
Too many difficulties led some people to opt for a trowellable, and subsequently gunnable, over-lining at some added costs.

Gunnable Lining


Gunned linings are said to have been commercially started in Japan to overcome some of the problems of bricked linings. Initially these were alumino-silicate based and later converted to magnesite based or basic type to assist with metallurgical practice. Although it provided a monolithic joint-free structure and relatively improved deskulling but little was gained in the way of preheat times or heat losses due to the relatively high density of the gunned linings. There was still a tendency for the linings to crack and spall during rapid preheat and this also precluded the use of gunned linings for cold start practices.

Advantages:
  • low risk of H picking by molten steel
  • no sand
  • low inventory
  • no joints
  • less labour intensive
  • relatively easy installation in lesser time
  • relatively less difficult to deskull
Disadvantages:
  • intensive curing required
  • high wastage because of rebound losses
  • poor insulation
  • “Cold Start” not possible
  • high washout risk
  • low thermal stability
  • dust problems
  • energy intensive
  • long T/D cycle
  • high costs
  • investment in equipment

Tundish Board Lining


The mid 1970’s saw the introduction of a new type of tundish wear lining; which were board systems comprising low density, highly insulating, disposable, pre-formed, and pre-cured refractory boards. Easy deskull, no equipment investment and the low cost of silica variety also contributed to its run-away popularity among many steel makers. FOSECO’s GARNEX became a household name in Indian continuous casting circles during this time. Initially silica based boards were used which allowed only “cold start” practice. Magnesite based boards were introduced in mid 1980’s to fulfill the requirement of pre-heatability, i.e., a “hot start” practice for low hydrogen considerations in the manufacture of high alloy quality steels. However, the labour intensiveness, presence of joints and sand backing, and breakages etc remained as inherent handicaps of board system.

Advantages:
  • low risk of H picking (when hot)
  • uniform liner shape
  • no need to cure
  • good insulation
  • cold start possible
  • easy deskull
  • low energy requirement
  • short T/D cycle
  • no investment in equipments
  • low washout risk
  • low cost (silica-based board)
Disadvantages:
  • joints
  • sand backing
  • hydrogen picking risk (when cold)
  • labour intensive
  • high inventory
  • handling/breakage problem
  • high cost (magnesite-based boards)
However, board system is still popular in countries where labour costs are low and application technologies are not readily available.

Sprayable Lining


Because of some of the above difficulties there was already a push towards automation of the tundish lining system. Meanwhile, advances in machine design and chemical formulation technology in advanced countries led to the development of a “Spray” system, in which a thick slurry could be transported after through mixing, and finally deposited onto the tundish after “atomizing” with compressed air. The first robotic application system was commissioned in 1982 which from the later half of the 1980’s started to be widely used in developed countries due to the significant benefits of lower placed density and better control of the lining thickness than gunned linings. This was no longer required to transfer the dry powder after fluidization (as required in gunning). This enabled the addition of fibers and other chemicals to the mass and homogeneous mixing and deposition became a reality. The lining could be preheated and the cast taken in a “hot start” fashion, or allowed to cool to room temperature and taken as a “cold start” tundish. While curing, it needs to be controlled to ensure lining integrity and this demands that the tundish permanent lining is ideally below 100 degrees C for satisfactory placement. Wet processes such as sprayable lining with up to 30% water addition by weight and the presence of hoses and spills may create OH and S issues in the steel plant. Even then this spray lining system was able to successfully combine many of the advantages of board and gunning, while eliminating the disadvantages like - joints, sand backing, rebound losses, dust problems, poor insulation etc.

Advantages:
  • low risk of H picking
  • no joints
  • no sand
  • low inventory
  • less labour intensive
  • easy deskull
  • good insulation
  • “cold start” also possible
  • controllable lining thickness
  • robotic application for big size tundish (involve large investment)
Disadvantages:
  • investment in equipments
  • intensive curing required
  • moderate washout risk
  • relatively longer T/D cycle (than boards)

Tundish - Dry Lining


Dry linings were introduced in Europe probably in 1986. The system differ from all previous processes in the sense that it is applied in a dry powder form and do not require the addition of water to form the tundish working lining. Generally it utilizes a resinous bond (Binder / Catalyst reaction) which is activated by relatively low amounts of heat (around 160OC). Vibration may or may not be required, depending upon the product being used, but it is essential to use a former and the dry powder is fed in the gap between the tundish permanent lining and the former. Adjacent figure shows the typical arrangement dry tundish lining curing system (schematic diagram). The hot air is introduced at approximately 400OC and the heating cycle takes around 45 minutes with further 30minutes for cooling. Thus a lot time can be saved while on the negative side; the dry system still has lower insulation (due to higher density) and is dependant on crainage in the tundish bay for installation.

Advantages:Refractory Technology: Schematic Diagram of Dry Tundish Lining Curing Arrangement
  • no joints
  • no sand
  • low H risk (when hot)
  • low inventory
  • less labour intensive
  • reduced tundish preparing time
  • low washout risk
  • easy deskull
  • uniform liner
  • clean environment friendly application
  • high sequence possible
  • OH and S benefits
  • easy, quick installation
  • improved steel cleanliness because of lining integrity
Disadvantages:
  • investment in equipments
  • H risk (when cold)
  • lower insulation
  • crainage dependence

So, while there are advantages even in bricked and gunning systems, the disadvantages outweigh the benefits. Similarly although there are some disadvantages in all the systems of board, spray or dry lining, the advantages seem to be more in these systems. Making a choice appears to be difficult amongst the three systems with advantages and disadvantages being almost equally balanced. Therefore, recourse must be taken of other factors like those of steel plant operations, quality of steel, etc when trying to decide between board, spray and dry linings.

See other related articles

A Comparative Evaluation of Different Types of Tundish Lining Refractories [Read]

Gunning and Spraying : Fundamental Differences in these two methods of Tundish Wear Lining [Read]