Tundish and its Role in Continuous Casting of Steel
Tundish and its Role in Continuous Casting of Steel
Continuous casting of steel is a widely used process and is an important step in the production of steel. The share of continuously cast steel around the world has increased significantly since the introduction of the continuous casting process in 1950s. Presently this share is around 97 %. However, concurrent with this increase in the use of the process, there are stringent quality requirements which have become crucial in the face of progressively increasing throughputs of the continuous casting machines and larger dimensions of the cast products.
In the continuous casting process, for the transfer of the liquid steel from a steel teeming ladle to the mould, an intermediate vessel, called a tundish, is used. The tundish is located above the mould, to receive the liquid steel from steel teeming ladle and to feed it to the mould at a regulated rate. It is needed to deliver the liquid steel to the moulds evenly and at a designed throughput rate and temperature without causing contamination by inclusions. The liquid steel flows out of the ladle into the tundish which links the discontinuous secondary metallurgy processes with the continuous casting process.
Tundish 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 performs the important role of serving as a buffer vessel between the batch ladle process and the continuous casting process. It is also the last metallurgical vessel before continuous casting and hence, it plays an essential role in delivering steel with the correct composition, temperature, and quality. This function has become increasingly important over the last few decades with increasingly stringent requirements for the quality of the steel products.
The contributions of the tundish in the process of continuous casting are (i) to reach stability of the liquid steel streams entering the casting mould, and in turn, to achieve a constant casting speed, (ii) to cast a sequence of heats, (iii) to change over the empty steel teeming ladle for a full steel teeming ladle without interrupting the flow of liquid steel in the moulds, (iv) to make a mixed grade with steel from two different grades of two different heats, if needed, (v) to provide possibility to prevent inclusions and slag from entering tundish and thus slipping into mould, (vi) to enhance oxide inclusion separation, (vii) to maintain a steady liquid steel height above the nozzles to the moulds, thereby keeping steel flow constant and hence casting speed constant as well, and (viii) to provide more stable stream patterns to the moulds.
The main function of the tundish is to be a steel reservoir between the steel teeming ladle and the mould, and in the case of multi-strand continuous casting machines to distribute the liquid steel into the different moulds. Tundish 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. 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.
Tundish is a refractory-lined vessel with a variety of possible geometries. It is a rectangular big end up refractory lined open container which can have a refractory lined cover on the top. There are several types and shapes of tundish. Tundishes are normally of elongated and geometrically simple shape. The shape of the tundish is typically rectangular, but delta, and ‘T’ shapes are also sometimes used. One common tundish design for multi-strands billet and bloom continuous casting machines is a trough shape with a pouring box offset at the midpoint. For the slab continuous casting machines the tundish is normally a short box or of a tub shape.
Tundish is designed to deliver the liquid steel at a designed output rate without major fluctuations in the flow. The flow rate is primarily controlled by the depth of the melt. Further control of the outlet flow can be performed by either stopper rods or slide gates. The number of outlets depends on the type of casting performed. The number of moulds to which the tundish delver the liquid steel is normally 1 to 2 for a slab continuous casting machine, 2 to 6 for a bloom continuous casting machine and 2 to 8 for a billet continuous casting machine.
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 mould(s) of a continuous casting machine, so as to avoid splashing and give a smoother flow. The delivery rate of liquid steel into the mould is held constant by keeping the depth of the liquid steel in the tundish constant.
A tundish is frequently divided into two sections. The first section is called inlet section which normally 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. The pouring stream from the ladle is directed downward to a position in the tundish bottom which is protected with a wear resistant impact pad. This position is normally as far as possible from the tundish nozzle to minimize turbulence. In other locations, the tundish is lined with refractory lining.
The tundish provides a continuous flow of liquid steel and acts as a secondary refining device. The flow behaviour in the continuous casting tundish dominates the quality and cleanliness of steel production. The refinement of liquid steel in tundish is carried through flotation on non-metallic inclusions at the upper surface during its residence. These floated inclusions are removed over from the surface. The tundish fluid flow plays an essential role in controlling the inclusion removal and residence time calculation. For achieving the optimum flow characteristics of the tundish, the flow control devices are deployed.
Different flow control devices 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. The flow control devices also reduce the detrimental effects of turbulence on the liquid steel surface, the liquid steel streams entering the mould, and the dead regions.
Tundish flow optimization is necessary with respect to size, shape, wear, and baffles and for the prediction of the mixing time. The benefits of the tundish flow optimization are (i) improving the mixing and homogenization time, (ii) identification of inactive flow regions (dead water), (iii) regions of prolonging refractory wear, (iv) separation of non-metallic inclusions, and (v) control of temperature stratification.
Nozzles for protecting the pouring stream against reoxidation between ladle and tundish, and tundish and mould, are used nowadays almost on all the continuous casting machines, at least when casting of high grade steels. They are located along its bottom for the distribution of liquid steel to the moulds. Both stopper controlled nozzles and slide gates of various designs are used to control the steel flow from the ladle to the tundish and from the tundish to the mould. The free surface of the liquid steel in tundish is normally covered with tundish flux to avoid reoxidation and heat losses from the liquid steel.
The discharge rate of liquid steel is controlled by the bore of the nozzle and the ferrostatic pressure (height of liquid steel in the tundish) above the nozzle. Different bores are selected depending on the section size being cast and casting speed needed. Stopper rod controlled nozzles are used for casting slabs and large sections when aluminum killed steels are produced. In this application, discharge rate of liquid steel through the nozzle is controlled manually or automatically by the setting of the stopper head in relation to the nozzle opening. Earlier oversized nozzles were used for casting aluminum killed steels because of the build-up of alumina so that the stopper head can be raised to compensate for a reduction in flow rate.
Recent developments in deoxidation practices together with the use of argon bubbling through the stopper head and nozzle units have minimized the alumina buildup problem. Another development in controlling liquid steel flow from the tundish is the application of slide gate systems which are similar to those employed on ladles. The slide gate system for the tundish normally consists of three plate type and can also provide the capability for changing nozzles during casting as well as changing nozzle size. Fig 1 shows a tundish in a continuous casting machine along with its components..
Fig 1 Continuous casting tundish and its components
Tundishes are normally preheated before the casting to minimize heat losses from the liquid steel during the initial stages of casting and thus avoid metal solidification, particularly in the critical nozzle areas. Tundish covers are also used to reduce radiant heat losses throughout the casting operation.
Tundish car is used for the transfer of a tundish. It is a self-propelled carriage which transports a tundish. Each tundish is heated to a high temperature by a heating apparatus at a standby position and is transported to the casting station by the tundish car immediately before casting begins. In addition to the travelling function, the tundish car comprises a lifting function for inserting a submerge nozzle arranged at the bottom of each tundish into a mould, a centering function for adjusting the position of the submerged nozzle, and an automatic liquid steel level control function for keeping the liquid steel at a constant level to separate impurities by flotation. The size of the tundish car is normally increased as the number of strands increases, and the interference with adjacent apparatuses becomes tighter. Hence, it is required to have a mechanism which is lean in terms of structure dynamics.
Tundish car is normally of half suspended design and is mounted at the main operating platform. It is normally hydraulic powered and is used to support and convey the tundish for casting or heating. One of the important devices of the automatic liquid steel level control is a weighing device, which is needed to measure the weight of the liquid steel in each tundish with high accuracy while the weight changes from time to time. Tundish car incorporates the weighing mechanism for the weight measurement to allow the weight of liquid steel to be continuously monitored.
From both the steady-state and non-steady state perspectives, the tundish is required to provide (i) sufficient volume to bridge ladle exchanges, (ii) an appropriate operating depth, (iii) uniform flow distribution to all strands, (iv) optimal residence time for inclusion flotation, (v) a quiet surface, (vi) thermal and chemical insulation, including appropriate refractories, and (vii) low drainage weight capability to optimize yield.
Tundish as a metallurgical reactor vessel
There is a constant demand for steel with improved properties, such as increased strength, ductility, durability, and corrosion resistance, which is needed for a large variety of applications. There is also the desire to make the steelmaking process more energy and cost efficient and to address environmental concerns. These issues has promoted the evolution of the tundish into a metallurgical reactor, with the function of performing final control over the properties of the melt before casting to obtain a final steel product with the desired mechanical properties.
Since the tundish is the final stage in the steelmaking process before casting, it also presents the last opportunity for the compositional control of the liquid steel. The main form of compositional control in the tundish is by limiting the number and size of non-metallic inclusions in the liquid steel when casting takes place.
During the transfer of liquid steel through the tundish, liquid steel interacts with the refractories, slag, and the atmosphere. With continuing emphasis on the quality of steel, it is now increasingly clear that the tundish has a far more important function as a continuous metallurgical reactor than originally envisaged. Hence, 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, flotation, 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’.
Over the years, there have been dramatic changes in the continuous casting tundish. From a mere reservoir and distribution vessel, the tundish today is viewed as a steel refining vessel. Tundish today also fulfills certain metallurgical functions such as feeding of the liquid steel to the mould at a controlled rate, and thermal and chemical homogenization etc. It also focuses on the continuous improvement of many quality related parameters such as fluid dynamics, thermal insulation, inclusion flotation and removal, and hydrogen pickup etc.
The continuous casting tundish has several roles which include (i) a critical link in the steelmaking chain of quality, (ii) a continuous refiner and (iii) a transmitter of metallurgical signals. The crux of the process remains that an uncontrolled tundish becomes a contaminator rather than a refiner. It is undeniable that the tundish is as much a part of clean steel practices as the ladle before it and the mould subsequent to it. If the quality built into the steel in the primary and secondary steelmaking pocesses is lost in the tundish, the ability to produce a quality product which meets the intended application depends entirely on recovery in the mould. This is a far more difficult task, considering the limited time that the steel resides in the confines of the mould, which provides a finite capability to clean the steel in the last stage prior to complete solidification.
The top surface of the tundish needs to be protected from the atmosphere. In most continuous casting machines, this is accomplished by the addition of a tundish flux layer. The tundish flux layer creates a surface slag. In addition to the need for inclusion absorption, the principal functions of the tundish slag layer are thermal insulation, chemical insulation, and buffering of ladle slag. Tundish slag coverings can be complex engineered multi-component chemical mixes such as basic fluxes ( lime / silica ratio greater than 2), or as simple as pure chemical insulating acid slags, such as burnt rice husk ashes or diatomaceous earth, both of which essentially consist of silica.
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, 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.
The 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 number and size of inclusions in the melt exiting the tundish are reduced by preventing the formation and growth of inclusions in the tundish, as well as by removing inclusions that are carried over from the ladle to the tundish. It is therefore also critical that the correct practices are followed during ladle operation to lower the amount of inclusions contained in the liquid steel being delivered to the tundish.
Another important function of the tundish is to control the temperature of the liquid steel delivered to the continuous casting process. Since pouring of the liquid steel from the ladle can take upto an hour, the temperature of the inlet stream changes with time. Coupled with the heat losses in the tundish itself, the temperature of the liquid steel being cast can hence be expected to fluctuate during a casting sequence. However, the liquid steel temperature influences the quality and properties of the product, casting machine operation, and refractory wear. It is hence desired to limit fluctuations and to keep the temperature as close to the optimal value as possible. Since the tundish operates as a buffer tank, fluctuations in the temperature of the liquid steel delivered to the casting machine can be reduced considerably if the mixing in the tundish is sufficient.
Several studies have been carried out on the tundishes in order to maximize the benefits of the residence time available for the flotation and assimilation of reaction products from the liquid steel into the slag phase. The products of reaction can be the products of deoxidation, reoxidation, precipitation, emulsification and / or entrainment of refractory components into the liquid steel, and thus encompass both indigenous and exogenous inclusions. Based on a sound choice of tundish design, the operation of a tundish needs to be geared for (i) to promote inclusion flotation by maximizing residence time, (ii) to ensure inclusion assimilation by a captive and non-corrosive slag, (iii) to reduce thermal and chemical losses from the liquid steel, (iv) to minimize short circuiting and dead regions, and (v) to offer the operator an optimal design for quality and yield.
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 hence 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 liquid steel against air absorption.
The basis for finding the solutions to the tundish metallurgy challenges, there are some basic requirements. These requirements include (i) use of a tundish size appropriate for the shop’s pacing and transition requirements, (ii) heats are to be sent on time with liquid steel at proper temperature, and properly cleaned, (iii) maximization of ladle free open performance, (iv) opening of the heats submerged and fully shrouded, (v) utilization of the automatic ladle slag detection and shut-off for avoiding easily reducible oxides in slags, linings and refractories, (vi) designing of the slags to meet the application requirements, (vii) ensuring the transfer systems are not subject to leaking or air aspiration, (viii) designing of the tundish flow control devices (including impact pads) to maximize flotation and minimize transitions, (ix) running of the tundish at its maximum volume during steady-state operations, (x) utilization of the technologies such as inert gas purging to minimize transient effects, (xi) monitoring of the temperature continuously, if possible, (xii) avoiding large temperature swings to maintain a stable tundish flow, (xiii) understanding and solving the root causes to the clogging problems, and, (xiv) maximizing of yield and productivity without jeopardizing the safety of people and the mould.
Tundish refractory lining
Tundish lining is another important part of the metallurgical system. The lining is ought to be inert and not to contribute to exogenous inclusions in the steel. Tundish refractories can be divided into two categories namely (i) lining refractories, and (ii) flow control devices.
Different refractories associated with tundish include tundish lining materials (both permanent and working lining), dams and weirs, impact pad, flow control system (mono-block stopper or slide gate), pouring stream protection between tundish and mould (shroud or submerged entry nozzle), tundish nozzle, and seating block. Dams and weirs are made of magnesite boards or alumina bricks. Liquid steel from tundish to mould is fed by nozzle submerged into the liquid steel in the mould. Submerged entry nozzles are to be resistant to corrosion and spalling. Nozzle clogging is also important. Isostatic pressed submerged entry nozzle with alumina graphite-fused silica are normally used.
The refractories in the tundish are necessary for uninterrupted, safe, and, of course, profitable quality production in the casting process. Sacrificial in nature, the refractories have a measurable cost and quality impact on the steel production process, and hence are to be controlled. In harnessing the flow and energy of the steel in the tundish, the refractories help to turn the tundish into a continuous refining vessel, both through their physical presence and by controlled chemical reactions.
The tundish lining material has a direct influence on the quality of the liquid steel, since it is close to the solidification stage. Tundish working lining remains in contact with steel and erosion is initiated at the liquid steel-air interface with fluctuation of liquid steel level in the tundish. Different metallic oxides present in the liquid steel are the primary eroding agents for the tundish refractory lining.
The refractory lining of a tundish has a defined life time depending on the qualities of the lining and the types of tundish slag. Chemical reaction between the refractory working lining layer and the tundish slag is very important especially in the case where there is high sequence casting.
It is desired that the tundish refractories fulfill a number of different functions which include resistance against liquid steel (high solidus temperature), resistance against tundish slag, low heat conductivity (good thermal isolation properties), good stability (no erosion of refractory during casting), high resistance to thermal shock, chemical inertness, need to be disposable after use, and reasonable in price. Further tundish refractories are to have low oxygen potential, good mechanical resistance, easy deskulling, and low hydrogen pick up by steel.
Whether refractories in the tundish contact steel directly or not, a wrong selection or application of these materials can have disastrous consequences, and can affect safety of the operating personnel. Paramount to protecting against the potential for steel penetration and eventual tundish breakouts is not only to select materials with the appropriate insulation, hot strength, and erosion-resistance properties, but also to avoid straight through joints. Steps in the refractories provide opportunities for steel to freeze, in case it penetrates, with the skulled steel providing some measure of retention of the overlying liquid.
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 centre’ 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 submerged entry nozzle), tundish nozzle, and seating block. For tundish lining there are a number of different lining practices. Fig 2 shows a typical tundish along with its refractories.
Fig 2 Typical tundish along with its refractories
The different tundish refractory lining practices can be categorized into five major types namely (i) brick lining, (ii) gunnable lining, (iii) tundish board lining, (iv) sprayable lining, and (v) dry lining.
Brick lining – Concept of refractory brick lining has been employed for tundish lining initially when the continuous casting was introduced in 1950s. 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 – Gunnable linings have been commercially started in Japan to overcome some of the problems associated with the 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 lining system. However, board lining system is popular in places where labour costs are low and application technologies are not readily available.
Sprayable lining – The development of sprayable lining has taken place 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 can 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, sprayable lining needs to be controlled to ensure lining integrity and this demand that the tundish permanent lining is ideally below 100 deg C for satisfactory placement. Wet processes such as sprayable lining with upto 30 % water addition by weight and the presence of hoses and spills can 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, and poor insulation etc.
Dry lining – Dry linings for tundish 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. Normally 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 around 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.