March 7, 2009

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]




2 comments:

  1. Thanks to Dr Joardar for interesting column. I'd add a couple of new tundish disposable lining technologies into the 5 row list made by Dr Joardar.

    1. A dry installed, inorganic coating for tundish. This technology was originally developed in Finland in the mid of 90's, now holding nearly 100% market share in Finland (4 plants, abt 12 000 tons consumption in 2008). This technology shows a big bundle of advantages like metal quality (low H, low oxide inclusion amount etc.), better heat balance (lower tapping temp. at furnace and at ladle, higher temp in tundish due to low thermal conductivity of coating material), energy savings (no cooling prior to mix installation, no mixing, no curing of the mix), quick put-through time of tundish (decreased fleet of tundishes), no vibration (sufficient strength is gained by mix own weight and by bond formation), lower investment cost (only simple metal form is needed with no electricity or pneumatics), lower maintenance and labor costs (no wearing parts), better process reliability (less sintering, easy desculling), no drainage needed, no waste mix formed, no fumes formed, much less CO2 formed (especially with olivine rich materials). Any aggregate (as MgO, olivine and others) is possible to apply. Also refractory cost in furnace/converter/ladle/tundish backing lining is decreased due to smaller thermal stress. The bond is carried out by the latent heat of backing lining (temperature from 200 to 450C or even higher is possible). If the tundish is cold, the backing lining can be heated with conventional methods (flame burner). Also curing the mix via the former is possible. As well, novel,ultra fast, simple (no electronics, no pneumatics needed) installation devices have been developed (4 units delivered to finnish steel plants).

    2. An earth moist tundish disposable lining was developed at he advent of 2. milleneum. A screw mixer is needed to mix abt 2-3% water (no chemicals) and the mix is simultaneously fed by the screw into the gap between the form and backing lining. The exothermic mix can be applied onto a cold tundish surface. This type of technology is applied at some swedish steelmills.

    ReplyDelete
  2. useful information, however we want more about latest products.Thanks

    ReplyDelete