Tundish and its role in Continuous Casting
Tundish and its role in Continuous Casting
Continuous casting of steel is a widely used process and an important step in the production of steel. The share of continuously cast steel around the world has increased significantly in the last 35 years or so. Presently this share is above 95 percent. However, concurrent with this increase in production levels, there are stringent quality requirements that have become crucial in the face of progressively increasing throughputs of continuous casting machines and larger dimensions of the cast products.
In the process of continuous casting, to transfer liquid steel from a steel teeming ladle to the mould, an intermediate vessel, called a tundish, is used. A tundish is a rectangular big end up refractory lined open container which may have a refractory lined cover on the top. The tundish bottom has one or more holes with slide gate(s) or stopper rod(s) for controlling the metal flow. It is used to feed liquid steel into the copper moulds of a continuous casting machine, so as to avoid splashing and give a smoother flow. The tundish being a reservoir of the liquid steel ensures the feed of the liquid steel to the continuous casting machine during the change of steel ladles, thus acting as a buffer of liquid steel. It smoothens out flow, regulates steel feed to the mould and cleans the metal. Metallic remains left inside a tundish are known as tundish skulls and need to be removed, typically by mechanical means (scraping, cutting). Scrap recovered in this way is ordinarily recycled in the steelmaking process.
The tundish is required to deliver the liquid steel to the moulds evenly and at a designed throughput rate and temperature without causing contamination by inclusions. The number of moulds is normally 1 to 2 for a slab casting machine, 2 to 4 for a bloom casting machine and 2 to 8 for a billet casting machine. The delivery rate of liquid steel into the mould is held constant by keeping the depth of the liquid steel in the tundish constant. Since the tundish act as a reservoir of liquid steel during the period of ladle change periods and since it continues to supply liquid steel to the moulds when the incoming liquid steel has stopped due to ladle change, it makes the sequence casting by a number of ladles feasible. Fig 1 shows a tundish in a continuous casting machine.
Fig 1 A tundish in a continuous casting machine
A tundish is often divided into two sections. The first section is called inlet section which generally has a pour box and where liquid steel is fed from the ladle. The second section is called outlet section from where liquid steel is fed into the mould. Different flow control arrangements such as dams, weirs, baffles with holes etc. are normally arranged along the length of the tundish. Longer path of liquid steel is preferred to prolong the residence time of liquid steel in the tundish to promote floatation of macro inclusions.
Tundish-a metallurgical vessel
The continuous casting tundish is the last metallurgical vessel through which molten metal flows before solidifying in the continuous casting mould. It serves as a buffer and links the discontinuous process of the secondary metallurgy in the ladle with the continuous casting process in the mould. During the transfer of liquid steel through the tundish, liquid steel interacts with refractories, slag, and the atmosphere. With continuing emphasis on the quality of steel, it is now increasingly clear that a tundish has a far more important function as a continuous reactor than originally envisaged. Thus, the proper design and operation of a tundish are important for delivering steel of strict composition and quality. A modern tundish is designed to provide maximum opportunity for carrying out various metallurgical operations such as inclusion separation, floatation, alloying, inclusion modification by calcium treatment, superheat control, thermal and composition homogenization, leading to the development of a separate area of secondary refining of steel, referred to as ‘Tundish Metallurgy’.
The tundish is seen as a contaminator of liquid steel. The main causes of inclusion formation and contamination of the liquid steel include deoxidation products, steel ladle lining erosion products and entrainment of ladle slag carried over from the ladle, entrainment of tundish slag by the excessive fluctuation especially at the inlet zone, re-oxidation of the steel by air in tundish, precipitation of inclusions at lower temperature such as TiO2 inclusions, erosion of tundish lining and emulsification of various slags into the liquid steel. Appreciable contamination normally occurs during transient periods of the sequential casting i.e. during ladle change at the transition of two heats.
These contaminations or inclusions are required to be floated out of the liquid steel during its flow through the tundish before the liquid steel is fed into the mould of the casting machine. Inclusions can be removed by the mechanisms which include (i) buoyancy rising and absorption to the top slag, (ii) fluid flow transport, (iii) argon gas bubble flotation, (iv) inclusion growth by collision and Ostwald-Ripening and floatation and (v) inclusion absorption to lining refractories. The final inclusion destination includes the top slag, the lining (safe removal) and mould (possible defects in the cast product if not removed in the mould).
The flow through a tundish is a hydrodynamic phenomenon. It includes the single phase turbulent fluid flow, multi phase fluid flow if the gas is injected from ladle shroud, residence time distribution, growth of inclusion with its motion and removal, mixing and grade transition, thermal energy transport and vortexing formation at the start and the end of the casting. The purpose of fluid flow optimization in the tundish is to achieve the best flow pattern to remove inclusions from the liquid steel. Flow optimization in the tundish can be achieved through the tundish shape and flow control devices such as turbulence inhibitors, impact pads, baffles, weirs and dams. A tundish is to be designed in a way as to realize an optimal flow and therefore higher cleanliness by providing (i) high average residence time, (ii) small severe turbulence, dead and short circuit volumes, (iii) large volume of laminar flow region, (iv) forced coagulation in suitable turbulent zones and floating of inclusions, assimilated by cover slag and (v) avoiding ‘open (red) eye’ creating uncovered surface of molten steel against air absorption.
Tundish refractory lining
The refractories used in tundish are required to have high stability and special properties. Tundish is one of the most important areas of refractory application and so, is also one of the biggest ‘cost control center’ in the continuous casting process. Various refractories associated with tundish are Tundish lining materials (permanent and working lining), dams and weirs, impact pad, flow control system (mono block stopper or tundish slide gate), pouring stream protection between tundish and mould (shroud or SEN), tundish nozzle and seating block. For tundish lining there are a number of different lining practices. These different practices can be categorized into the following 5 major types.
- Brick lining – Concept of refractory brick lining was employed for tundish lining initially when the continuous casting was introduced in 1960s. These linings were of high alumina bricks and were essentially an extension of ladle refractory practices to the tundish. There were a number of difficulties associated with this type of lining which led to the development of alternative lining practices.
- Gunnable lining practice – Gunnable linings have been commercially started in Japan to overcome some of the problems associated with brick lining. Initially these were alumino-silicate based and later converted to magnesite based or basic type to assist with metallurgical practice. This lining provided a monolithic joint-free structure and relatively improved deskulling but little was gained in the way of preheats 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. This also precluded the use of gunned linings for cold start practices.
- Tundish board lining – A new type of tundish wear lining was introduced in mid 1970s. This lining consisted of board systems comprising low density, highly insulating, disposable, pre-formed, and pre-cured refractory boards. Easy deskulling, no equipment investment and the low cost of silica variety also contributed to its run-away popularity among many steel makers. Initially silica based boards were used which allowed only ‘cold start’ practice. Magnesite based boards were introduced in mid 1980s 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. However, board system is popular in places where labour costs are low and application technologies are not readily available.
- Sprayable lining – The development of sprayable lining to overcome difficulties associated with other lining practices and to give a push towards the automation of the tundish lining system. In this sprayable lining system, 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 second half of the 1980s started to be widely used due to the significant benefits of lower placed density and better control of the lining thickness than gunned linings. It was no longer necessary 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’ mode, 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 deg 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 cause operational health and safety 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.
- 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 160 deg C). 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. The hot air is introduced at approximately 400 deg C and the heating cycle takes around 45 minutes with further 30 minutes for cooling. Thus a lot time can be saved. On the negative side, the dry system has lower insulation due to higher density.