Tundish Metallurgy
Tundish Metallurgy
For the transfer of liquid steel from a steel teeming ladle to the moulds of a continuous casting machine (CCM), an intermediate vessel, called a tundish, is used. The tundish is intended to deliver the liquid steel to the moulds evenly and at a designed throughput rate and temperature without causing its contamination by inclusions. The tundish acts as a reservoir during the ladle change periods and continues to supply the liquid steel to the moulds when incoming liquid steel is stopped, making possible sequential casting with a number of teeming ladles. The main causes for inclusion formation and contamination of the liquid steel include reoxidation of the liquid steel by air and carried over oxidizing ladle slag, entrainment of tundish and ladle slag, and emulsification of these slags into the liquid steel. These inclusions are required to be floated out of the liquid steel during its flow through the tundish before being teemed into the moulds.
Since the invention of the continuous casting process in the 1960s, outstanding progress has been made in the casting technology i.e. the continuous casting machine itself as well as controlling and optimizing metallurgical phenomena in the continuous casting process. The initial function of tundish has been to act as a ‘reservoir and distributor’ for liquid steel from the ladle into the moulds in a multi-strand casting machine. During the past decades also, the great potential of tundish has been s realized and its functions have been extended to improved control of steel temperature and chemistry.
Tundish as an element of the continuous casting machine plays an important role in the technological process of the continuous casting of the steel. In continuous casting process, tundish is used as an intermediate reservoir placed between the batch secondary steelmaking process and continuous casting moulds. It receives the liquid steel from the ladle and distributes it to the continuous casting moulds. In addition to its conventional role (as a reservoir and distributor), tundish is also used as a reactor for producing clean steel and assuring smooth operation of the casting machine.
The tundish is a big-end-up, refractory-lined vessel, which can have a refractory-lined lid on the top. The tundish bottom has one or more nozzle port(s) with stopper rod(s) or slide gate(s) for controlling the flow of liquid steel. The tundish is frequently divided into two sections namely (i) an inlet section, which normally has a pour box and where the liquid steel is fed from the ladle, and (ii) an outlet section from which liquid steel is fed into the mould(s). Different flow modifying devices (FMDs), such as dams, weirs, and baffles with holes, etc. can be arranged along the length of the tundish. The liquid steel flow path from inlet to outlet of the tundish is important. Longer path is preferred for prolonging the residence time of the liquid steel so as to promote the flotation of macro-inclusions.
In the tundish, the slag is less dense than the liquid steel and hence resides on the top of it. The slag provides a sink into which the non-metallic inclusions float to and dissolve. The slag also protects the liquid steel from air and heat loss. However, when slag is entrained in the liquid steel as a result of increased turbulence or shearing at the steel-slag interface, slag can also become a source of non-metallic inclusions.
The tundish is a refractory-lined vessel with a variety of possible geometries, but most commonly roughly rectangular in shape. It is designed to deliver liquid steel at a designed output rate without major fluctuations in the flow. The flow rate is mainly controlled by the depth of the liquid steel. 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 the cast product. Normally there are 1 to 2 moulds for a slab casting machine, 2 to 6 for a bloom casting machine, and 3 to 8 moulds for a billet casting machine.
Since the tundish is the final stage in the steelmaking process before continuous casting, it also presents the last opportunity for the control of the liquid steel composition. 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 the environmental concerns. These issues have promoted the evolution of the tundish into a metallurgical reactor, with the function of performing final control over the properties of the liquid steel before continuous casting to get a final steel product with the desired mechanical properties. Fig 1 gives the cross-section of a continuous casting machine where the tundish location is shown.
Fig 1 Cross-section of continuous casting machine
The continuous casting tundish serves as a buffer and links the discontinuous process of the secondary steelmaking in the ladle with the continuous casting process in the moulds. It acts as a reservoir during the ladle change periods and continues to supply liquid steel to the mould when incoming liquid steel is stopped, making sequential casting by a number of ladles possible. The main causes for inclusion formation and contamination of the liquid steel include reoxidation of the liquid steel by air and carried over oxidizing ladle slag, entrainment of tundish and ladle slag, and emulsification of these slags into the liquid steel. These inclusions are to be floated out of the liquid steel during its flow through the tundish before being teemed into the mould.
Steel flows from the ladle, through the tundish into the mould where it solidifies. Flow between vessels is driven by gravity. Between the tundish and mould, this driving force is proportional to the head of liquid steel between the top surface of the liquid steel in the tundish and the liquid steel level in the mould. Control of the flow rate into the mould is achieved by metering nozzles, stopper rods, or slide gates. The liquid steel flow transfer is a key operation of continuous casting which affects strongly solidified shell profile, floatation of the non-metallic inclusion, and mould flux entrainment by large meniscus level fluctuations.
Different technologies such as a long nozzle or an inert gas shrouding pipe have been implemented to reduce air reoxidation and slag emulsification. Similarly, liquid steel flow control devices have been used to improve flotation of inclusions formed during the process. Implementation of active control of liquid steel temperature in a tundish has also contributed to casting clean steel. These measures have proved to be quite successful, at least during the steady state tundish operation, but are not sufficient for the non-steady state operation. Non-steady state operation is an integral part of long sequential casting which is done for better metal yield. Although it is desirable to cast steel of high quality, a compromise between the quality and cost has always been struck in any tundish operation.
Without ladle furnace processing, the deoxidized liquid steel has macro-inclusions and a large number of micro-inclusions of indigenous origin which can agglomerate to form macro-inclusions during the liquid steel transfer. A tundish is able to reduce some fraction of macro-inclusions from the liquid steel, adjust chemical compositions, and control liquid steel temperature to an appropriate level for feeding into the mould. With the use of ladle furnace and / or vacuum degasser, steel cleanliness has considerably improved and the tundish is now seen more as a contaminating vessel than a refining vessel. Appreciable contamination normally occurs during transient periods (or non-steady state) of the sequential casting (i.e., during ladle opening, at the transition of two heats (or ladle change), and during ladle emptying, and during transient periods), the incoming liquid steel stream and any metal splash are heavily reoxidized by the ambient air and by the oxidizing ladle slag which is carried over into the tundish with the liquid steel.
The main procedure of the control of the composition in the tundish is by limiting the number and size of non-metallic inclusions in the liquid steel when casting takes place. Inclusions can cause problems during continuous casting, rolling, and heat-treating processes and can result in the failure of the steel product during its application. For example, inclusions can cause cracked flanges because of the lack of formability in LCAK (low carbon aluminum killed) steel cans and lower fatigue lives in axles and bearings. This is because of the fact that both fatigue life and formability is highly affected by oxide and sulphide inclusions. An example of problems caused during forming is sliver defects, which occur as lines parallel to the rolling direction along the steel grip surface. Slivers are problematic in LCAK steel sheets used in the automotive industry and originate from aluminates or complex non-metallic inclusions.
Another potential problem caused by inclusions is the clogging of both the nozzle into the tundish and the SEN (submerged entry nozzle) into the mould. This can have several detrimental effects on the steelmaking process. Firstly, clogs can dislodge from the nozzle and are either captured in the liquid steel or in the flux, changing its composition. Both of these cases can cause defects in the product. Clogs also change the flow patterns in the nozzle, which can disrupt flow in the mould. Also, flow rate control devices can cause interference with level control in the mould when compensating for the clog.
These problems tend to worsen with increased concentration of inclusions, as well as with the maximum size of inclusions present. For different applications, there are limits to the maximum size (termed the critical inclusion size), and number of inclusions. Steels with more demanding processing and applications normally have a lower critical inclusion size and number density.
The liquid steel stream hits and aggressively emulsifies the ladle slag and tundish slag floating on the liquid steel surface, which eventually get entrained into the liquid steel. Both the reoxidation and the slag entrainment generate harmful macro-oxide inclusions. The aluminum-deoxidized liquid steel, even after removal of large particles of deoxidation product in the ladle furnace contains a large number of suspended fine alumina particles. These particles are found to agglomerate by turbulent liquid steel flow during the liquid steel transfer from the ladle through the tundish to the mould, forming large alumina clusters. The macro-inclusions and large alumina clusters are known to be the major cause of down-stream processing problems and defects occurring in strands and their final products.
Design and operation of a tundish is to be directed toward minimizing the formation of the macro-inclusions and alumina clusters, and removing them once they form. Otherwise, all the effort made in cleaning the liquid steel in the ladle furnace and during other process steps has no value.
Continuous casting tundishes normally have multiple outlets through which steel is continuously fed to the respective moulds at the constant rate. In case of strand break-out or non-availability of liquid steel, a particular strand is closed, which causes increasing the casting duration of the liquid steel in the ladle. Closing the outlet in multi-strand tundish alters the flow behaviour inside the tundish and hence the effectiveness of tundish with regard to its residence time distribution (RTD) behaviour is liable to be changed.
The tundish being the last metallurgical vessel before continuous casting, plays an essential role in delivering steel with the correct composition, quality, and temperature to the moulds. This function has become increasingly important over the last couple of decades with increasingly stringent requirements for the quality of steel products.
The knowledge of the temperature of the liquid steel poured from the ladle is relevant to get high quality products. High super-heat above liquidus temperature in the tundish increases central segregation, affect grain size, and even produce break-outs owing to local solidified shell thinning of cast products which interrupt the continuous casting sequence. On the other hand, low super-heat in the tundish promotes clogging of tundish nozzles, causes macro-inclusions entrapment, affects flux powder melting, and increases the probability of mould sticker formation.
After tapping of the liquid steel, it is held in the ladle and is transferred to the ladle furnace and trimming station for final adjustment of the grade composition and temperature. The ladle is then delivered to continuous casting machine and no further temperature correction is possible. The forecast of temperature of the liquid steel to be poured in the tundish depends upon several operating parameters such as the thermal history and wear of the tundish refractory lining, precise knowledge of refractory thermal properties and transfer coefficients, the use of tundish covers and insulating slag layers, the time of each operating stage lasts, and the teeming rate.
The fundamental aspects of the solutions to the tundish metallurgy challenges are based on several basic principles namely (i) use of a tundish size appropriate for the steel melting shop’s spacing and transition requirements, (ii) sending of heats on time, in temperature, and with properly cleaned liquid steel, (iii) maximization of ladle free open performance, (iv) opening of the heats submerged and fully shrouded, (v) utilization of automatic ladle slag detection and shut-off, (vi) avoiding of easily reducible oxides in slags, linings, and refractories, (vii) designing of slags to meet the application requirements, (viii) ensure that the transfer systems are not subject to leaking or air aspiration, (vii) designing of the tundish flow modifying devices, including impact pads for maximizing flotation and minimizing transitions, (viii) running of the tundish at its maximum volume during steady-state operations, (ix) utilizing technologies such as inert gas purging for minimizing transient effects, (x) monitoring of the temperature continuously, if possible, (xi) avoiding large temperature swings for maintaining a stable tundish flow, (xii) understanding and solving of the root causes to the clogging problems, (xiii) maximizing of the yield and productivity, and (xiv) not jeopardizing the people (safety is always to be the first objective) or the mould.
The tundish serves as an important metallurgical reaction vessel, where quality can be improved, maintained, or lost. The flow objective in the tundish is to encourage uniformity and inclusion removal, while avoiding flow-related issues. Tundish flow issues include surface turbulence, short circuiting, dead zones, and vortexing. Excessive flow directed across the top surface can produce turbulence and lead to reoxidation and slag entrainment. Short circuiting allows incoming liquid steel from the ladle to exit prematurely into the mould with insufficient time for inclusion flotation. Dead zones are stagnant, colder regions which prevent inclusion removal and can slowly mix and contaminate the new steel flowing through the tundish. If the liquid level is too shallow, high-speed, asymmetric flow can produce vortexing, which can entrain surface slag down into the mould.
Flow behaviour in the tundish is governed mainly by the size and shape of the vessel and the location of flow modifying devices such as dams and weirs. The flow pattern is also affected by the liquid steel flow rate and its temperature distribution. Thermal buoyancy tends to lift up the hotter, lower-density flowing steel, while colder steel tends to flow down the walls and along the bottom. A temperature difference of only a few degrees is enough to lift the jet flowing beneath the weir and completely reverse the flow direction in the second chamber of the tundish. Flow in the tundish is also highly affected by the ladle-tundish nozzle geometry and argon (Ar) gas in the ladle stream. Problems related to surface turbulence can be reduced by avoiding excessive argon levels in the ladle stream and by using fully shrouded and immersed nozzles.
Historically, tundish design concepts have been geared toward steady-state operation rather than transitional, or non-steady-state issues. Today, the functions of the tundish include more than the basic purpose of acting as a buffer reservoir between the ladle and the mould(s). Tundishes also carry the requirements for effective inclusion removal, thermal equilibration, chemical homogenization, and provision of a non-reactive container environment which supports the kinetics and thermodynamics of these functions. A proper basic design is critical in order for the tundish to be used as an inclusion, temperature, and chemistry refining vessel, or at least have this vessel be a non-contaminator in these phenomena.
In order to improve steel quality and to achieve a better flow control, design of the casting channel geometry, in particular in the flow regulation region, is critical. The flow rate is controlled by restricting the opening with either a stopper rod comparable to a needle valve or a tundish slide gate. There is a wide variety of stopper nose geometries and throttling plate diameters with different degrees of sensitivity and controllability of steel flow. Dimensions of the casting channel, the geometry of both the stopper nose / the nozzle seat or of the tundish nozzle / the throttling plate, and the argon gas injection, influence the flow control performance.
The simplest means to control the flow rate is to choose the appropriate size of the opening in the bottom of the tundish which restricts the flow to the desired rate. This inexpensive method is used in conjunction with open stream pouring of lower quality steel, where some air entrainment can be tolerated. Air entrainment can be reduced, but not avoided, by surrounding the stream with argon gas through one of the shrouding methods. Of different choices available, the bellows appears to be very effective.
Metering nozzles have the disadvantage that variations of the liquid level in the tundish produce variations in flow rate which is to be compensated by changes in withdrawal rate of the strand in order to maintain a constant average liquid level in the mould. A second concern is that the condition of the stream is very sensitive to the flow pattern in the tundish and to any imperfections in the nozzle shape, such as oxide build-up or notches in the outside of the nozzle lip. The condition of the stream is important in open pouring since it directly affects the quality of the final steel product.
There is the possibility of two different stream conditions. A rough, turbulent stream is to be avoided since (i) its irregular shape entrains much more air, leading to more and larger oxide inclusions in the product, and (ii) it creates fluctuations in the liquid level in the mould when it impacts the surface, leading to non-uniform solidification, surface defects, and even break-outs. A tight, smooth stream entrains less air and creates less disturbance of the meniscus region in thew mould while it penetrates deeper below the liquid surface. Tight streams are hence very desirable. The funnel helps to improve surface quality by substantially lowering meniscus turbulence, although it has little effect on air entrainment.
For avoiding reoxidation and produce higher quality steel, a protective ceramic nozzle can be used to shroud the flow of steel into the mould. The flow rate is controlled by restricting the opening with either a stopper rod, or a slide gate. Flow control with a stopper rod is slightly more difficult than with slide gates since the stopper rod is to be manipulated through the entire depth of the liquid steel in the tundish. The area of the annular opening which controls the flow is more sensitive to displacement. In addition, a continuous nozzle does not allow fast exchange of SEN tubes and needs some other means for emergency flow stoppage. However, the stopper rod offers several significant advantages over slide gates namely (i) natural prevention of liquid steel from entering the upper tundish well and freezing prior to start-up without the need for special flow modifying devices, (ii) natural prevention of vortex formation above the tundish well and possible slag entrainment into the nozzle when the liquid level is low, (iii) easier sealing to avoid air entrainment because of the reduced number of moving surfaces, and (iv) more uniform distribution of flow to both ports, so that flow entering the mould cavity is more symmetrical.
When selecting the overall tundish design for a particular steelmaking operation, the needs and concerns of the steelmaker which are to be addressed are (i) consideration of the steel product(s) to be cast, (ii) matching the number of strands to the pacing of the steel melting shop, (iii) cleanliness improvements, (iv) lining thickness and configuration, (v) necessity for costly tundish flow modifying devices, (v) tundish fly capability to support extended continuous casting, and (vi) yield losses. All of these issues are of importance to the steelmaker, but the issue of improving quality in the tundish perhaps appears to be the largest.
Tundish size is defined by its wall angles and the length, height, and width dimensions, while geometry is frequently dictated not only by the number of strands to be cast, but also by available shop floor layout constraints. Fig 2 shows examples of different types of tundish configurations.
Fig 2 Types of tundish configurations
Fig 2a and Fig 2b show a number of basic tundish designs. The trough-shaped tundish, alternately referred to as a boat-shape or bath-tub-shape, is probably the most frequently encountered design. Variations of the rectangular-base trough include coffin-shaped and flared-trough shaped designs. The trough shape is normally encountered in single-strand and two-strand slab casting machines, and is also found on several bloom and billet casting machines. The V-shaped tundish is more prevalent in Europe than in either America or the eastern part of the world, offering a larger ladle stream pour box and a more circuitous path to the strand than a conventional trough tundish. The C-tundish is a combination of the boat-shaped tundish and V-shaped tundish. The T-shaped tundish is encountered principally on bloom / billet casting machines, and is essentially a modified trough with a separate and normally central pour box region. This design, while minimizing turbulence impacts to the inner strands and ensuring a full liquid steel head for delivery to all strands, is prone to short-circuiting issues. More modern bloom and billet casting machines have relied on the delta-shape with either a ‘V’ or flat wall near the ladle stream impact to synchronize flow to all strands.
Compared to the rectangular trough shapes, these trapezoidal tundishes have the advantage of simpler mould access and afford more protection for the strand operators and stand equipment, as the mould panels are normally farther removed from the ladle shroud (pour box) than in a trough tundish. However, trapezoidal tundishes also have shortcomings when compared to trough shaped tundishes in terms of increased refractory costs, tundish skull weights, size of flow modifying devices, and heat losses because of the higher quantity of surface area exposure.
One of the newer design evolutions is the H-shaped tundish. This type of design is shown in Fig 2c. The H-shaped design essentially consists of linked troughs, allowing the bridging of a ladle exchange period without the drop in tundish weight experienced in conventional tundishes. In a normal operation, a single set of hydraulics is used to manipulate the ladle gates from the tundish platform, such that the teeming rate drops to zero during the ladle change. Once the new heat has been connected and the ladle has been opened, the teeming rate is increased beyond the steady-state rate in order to refill the tundish back to its normal operating level as quickly as possible. Once this level is reached, the teeming rate is throttled back to match the casting rate in the mould(s).
In the H-shaped tundish, separate hydraulics allow two ladles to be poured into the tundish simultaneously, and the teeming rates of the old and new heat are matched to equal the output rate of the casting machine strand(s), such that tundish level remains constant. Different studies on interstitial free and drawn-and-ironed grades cast with such a tundish suggest that the quality of the ladle exchange portion is comparable to that cast under steady-state conditions in the middle of a ladle. A further benefit of the H-shaped tundish is the elimination of thermal events associated with heat losses through the tundish walls, as can be induced by large tundish level fluctuations.
Flotation of larger inclusions is normally adequate for all tundishes, although small inclusions (less than 15 micro-meters) do not tend to be floated effectively in any tundish configuration. However, there exist opposing views with respect to the issue of tundish operating depth. It is frequently stated that longer tundishes with lower operating heights yield more efficient inclusions removal. It is certainly true that shallower designs allow better separation, as the relative distance to be covered by an inclusion during Stokes’ law flotation from the liquid steel to the receptive slag interface is smaller on an average.
A shallower tundish also provides better chemical transition compared to the tundishes of equal volume but higher operating depth, owing to the mixing behaviour of the streams. However, a shallower operating depth also have shortcomings in terms of a reduced ability to dissipate turbulent ladle stream-energy and a higher likelihood of top slag vortexing from the tundish into the mould. Hence, the designer is to balance these issues prior to settling on a final tundish geometry solution.
After determining the appropriate shape for the application, two key points for the tundish designer are what to incorporate into the basic tundish and how to address situations which can potentially lead to problems during tundish operations. From both the steady-state and non-steady-state perspectives, a given tundish solution is needed 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 calm surface, (vi) thermal and chemical insulation, including appropriate refractories, and (vii) low drainage weight capability to optimize yield.
There is no universal tundish solution which allows flotation of all inclusions. Since no basic tundish shell design is sufficiently effective, protection during transfer operations and metallurgical improvements are a necessity for any tundish solution. Fig 3 provides a schematic outline of the functions carried out in such a tundish. These solutions seldom are all applied to a given tundish, and the operator is required to balance the performance with the cost.
Fig 3 Schematic outline of the functions accomplished in a tundish
Protection of the liquid steel stream from the ladle to the tundish, an absolute necessity in all aluminum (Al) killed carbon and specialty steel grades, is normally achieved by means of combined refractory and gas shrouding. Early shrouding systems consisted of gas blanketing only, but advances in refractory reliability led to the emergence of physical shrouding systems. Initially, fused silica (SiO2) ladle shrouds have prevailed, but these are prone to chemical and physical attack by the liquid steel teeming stream. Over the past decade, alumina-graphite (ALG) refractories have emerged as the standard solution to shrouding in tundish operations. Alumina-graphite offers good thermal shock resistance and resistance to metal and slag attack, though the latter is being addressed with more resistant materials such as nitrides or zirconia. In order to prevent aspiration through the shroud walls, sufficient refractory thickness and density, as well as low permeability, are needed. Oxidation is controlled by application of glazes on the interior and exterior surfaces.
The in-service connection of the ladle shroud to the collector nozzle on the ladle is frequently improved by means of a refractory gasket, which can be a sticky (or gummy) dough-nut, a semi-permeable ring, or an impervious ceramic. Argon gas can be injected into the shroud, into the bayonet seal can of the shroud, and / or introduced using a permeable ring for blanketing the connection and hence avoiding air entrainment. When injected into the ladle shroud, argon provides a positive pressure along with the column of steel to minimize air aspiration at the joints. Gas flow rates vary from shop to shop, frequently initially recommended by the refractory suppliers and then ‘tweaked’ by operations personnel based on nitrogen pick-up performance.
The objective of all these systems is zero aspiration. Typical obstructions to this are crushed gaskets, poor vertical ladle shroud alignment, and cracks in the refractory. Very good numbers for ladle-tundish nitrogen pick-up are of the order of the detectability limit for nitrogen, and have been reported as low as 2 ppm (parts per millions).
Nitrogen pick-up from ladle to tundish can also be affected by turbulence in the pour box, where submerged ladle opening practices and turbulence suppression shapes have been shown to yield quality benefits. Unsubmerged ladle opening practice not only forces vigorous slag / metal interactions, leading to increased inclusion formation, but also provides exposure of the liquid steel in the falling stream as well as in the tundish pour box to the atmosphere. A submerged opening practice does need a shroud with a larger diameter at the bottom than at the top to prevent so-called blow-backs during ladle opening, induced by resistance from constrictions. This can be achieved with either bell type shrouds (Fig 4a) or reverse-taper shrouds, both of which have an enlarged diameter in the submerged portion. Multipliers of the order of 1.25 times the upper diameter are frequently used in designing linear reverse-taper shrouds, which tend to weigh less and can be manipulated more easily than bell shrouds.
Fig 4 Bell type ladle shroud and tundish with stopper rod control
If the turbulent energy in the pour box can be harnessed, the steelmaker can then focus on metallurgical measures to improve inclusion flotation in the tundish. Principals among these are flow modifying devices known as dams, weirs, and baffles, which serve to assist in directing inclusions upward to a captive tundish slag layer.
Argon bubbling, using a concept frequently referred to as passing the liquid steel flow through a ‘curtain of bubbles’, can be used to improve the natural particle rise velocity by combining the inclusions with insoluble gas bubbles. The rise rate is accelerated because of the much higher buoyancy of the combined gas / inclusions particles. The bubbling can be affected either through porous plugs in the bottom of the tundish, or by providing gas purging through the top of a ‘hockey-stick’ dam.
Although a curtain of bubbles appears to be theoretically advantageous, neither method to induce such behaviour in the tundish appears to be used extensively in practice. Some experiments have cast doubt on whether fine bubbles actually persist in the liquid steel, since bubble coalescence at the injection point appears to be a significant mechanism inhibiting the release of fine bubbles. However, it is to be noted that use of bubbling devices in the tundish does need attention to gas pressure and volume to prevent ‘boiling’ and ‘open eyes’, which are manifestations of both slag / metal mixing and reactions with the atmosphere.
Tundish lining is another important part of the metallurgical system. The lining ought to be inert and not to contribute to exogenous inclusions in the liquid steel. Applications are being developed where the tundish lining is designed to actually react with solid inclusions in the liquid steel to form liquid compounds which do not clog. Equally important is the control of reducible components in the lining, such as silica and alkali oxides. If uncontrolled, these can be leached out during the casting process and contribute to formation of alumina inclusions, as the dissolved aluminum in the liquid steel reacts with the more reducible oxide species at the tundish wall.
The role of tundish has been originally to function as a steel reservoir after the ladle and a distributor into continuous casting moulds. It has been designed to get proper liquid steel flow pattern through the tundish and to keep thermal losses inside certain limits. When production of ‘clean’ steels with high requirements for oxide inclusions has been developed by applying sophisticated secondary steelmaking treatments in the ladle, the role of tundish became more critical. It became essential to maintain or even to improve the level of the steel cleanliness by minimizing contamination of steel with air, slag, or refractory materials between ladle and the casting moulds.
As per a new principle of clean steel production, the main function of tundish is to maintain the level of cleanliness achieved in the ladle by minimizing harmful contamination of steel with air, slag, or refractory materials from ladle to moulds. This is called a ‘protective or inert tundish practice’. As a result, the function of tundish slag is a barrier against reoxidation and thermal losses. By reconstructing tundish to a closed chamber with lid, argon gas shrouding, and eventual heating system, tundish slag can be even ignored. The concept of ‘active tundish slag’ aims definitely at improving of steel cleanliness by ‘tailoring’ a proper tundish slag.
The top surface of the tundish needs to be protected from the atmosphere. In the majority of the continuous casting machines, this is done by the addition of a tundish flux layer. A few casting machines around the world are equipped with fully sealed lids which can sustain an inert gas atmosphere above the liquid steel. In these cases, tundish flux is not added, and atmospheric control is maintained by means of oxygen sensors. 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.
As is common in steelmaking practices, slags are designed for the vessel in which they reside and tend to be diminished in effectiveness by carry-over slag from the previous operation. Tundish slag coverings can be complex engineered multi-component chemical mixes such as basic fluxes (‘basic’ refers to a lime-silica ratio or V-ratio higher 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 solution for a tundish depends on the quantity of refining which is intended and the costs which can be justified through quality improvements.
While acid fluxes containing several percent of carbon make excellent insulators, they do little to provide for steel cleanliness improvements. Basic fluxes, which provide excellent alumina (Al2O3) absorption, are much poorer insulators and are prone to crusting, prevalently when tundish life exceeds 10 heats. Some steel melting shops utilize a two-flux practice, where an efficient inclusion absorber is placed on the tundish first and then is augmented periodically with a highly insulating top cover.
The provision of a fluid basic slag in contact with the liquid steel improves cleanliness, as measured by total oxygen (O2) data, and yields a reduction in both the average and the deviation of oxide counts. However, it needs to be recognized that it is difficult to replenish the layer in contact with the liquid steel when a different insulating powder is used over top. In this scenario, the fluxes combine and continually lower the basicity of the inclusion-absorbing layer throughout the tundish lining campaign. Hence, tundish covering slag composition is to be suited to purpose, and the flux has to be adapted to prevalent conditions. For example, if no physical cover is available, then the tundish flux is required to carry a high insulation value, frequently to the detriment of inclusion absorption capability. If the function of the tundish slag is to provide substantial refining over longer sequences, then control of reducible oxides, including silica, is supreme. Fig 5a and Fig 5b provide laboratory data in support of this argument.
Fig 5 Some of the chemical relationships in tundish
Here, the lime-silica ratio has been modified as the ratio of lime (CaO) to combined silica and alumina units to account for the amphoteric behaviour of alumina, which means that alumina acts as an acid in a basic environment and vice-versa. The capability of slag to absorb alumina is clearly a function of modified V-ratio as well as initial acid component content. A second examination of the graphs suggests that, for the three slags (used in the experiment), the modified V-ratio has a far greater influence on inclusion absorption behaviour than initial silica content.
Similar behaviour is been observed in specialty steels. As shown in Fig 5c, tundish slag basicity, as expressed by conventional V-ratio, is critical in avoiding loss of valuable alloying constituents, the example being measured on liquid steels of American Iron and Steel Institute (AISI) 300-series stainless steel. The effect of increasing tundish slag basicity on reducible oxide analysis is quite dramatic, i.e., a change in basicity from V = 0.5 to V = 2 results in an order of magnitude reduction in oxidation behaviour.
There are several methods of gauging the effectiveness of a tundish slag. The most common techniques involve slag oxide component analyses through-out a tundish campaign and determining the changes in alumina, V-ratio, and reducible oxide contents. Caution is needed to be exercised in terms of consistent sampling location as well as in the interpretation of the data, since ladle slag carry-over contamination can considerably impact the results. However, the steelmaker is not to under-appreciate this method of assessing the slag in the tundish, as this type of analysis can help to determine when excessive slag carry-over into the pour box has occurred. Prior to the arrival of automatic slag detection methods, this has been a common method to determine whether prime cast product quality is affected because of the ladle slag carry-over.
Other methods include assessment of fadable liquid steel alloying constituents such as aluminum (Al) or titanium (Ti), which are related to steel cleanliness. Fade is defined as a loss in alloying constituent from the last ladle analysis to the tundish test. A suitable basic tundish flux reduces the fade, while a more acidic flux can contribute to higher-than-desired aluminum losses because of the reactions between the silica and the dissolved aluminum in the liquid steel. The inverse method is also used by some steelmakers who measure a rise in dissolved silicon (Si) because of the slag / metal reactions in the tundish.
Nitrogen pick-up is a poor indicator of slag effectiveness, as there exist numerous opportunities for aspiration which has little to do with the chemical shielding efficiency of the tundish slag, and hence can lead to erroneous conclusions. Proper assessment of metal fade or pick-up needs good sample integrity at both the ladle and tundish, with frequently, the former is less reliable.
One item to be cognizant of is that some steelmakers occasionally use bags of mould flux to break up crusting in the tundish, especially around the ladle shroud. This practice is to be discouraged, foremost for the fact which makes this practice successful in the first place, as mould powders contain a high degree of fluidizers such as alkali oxides and fluorine-bearing compounds. Both of these constituents are harmful to the refractory life, attacking the ladle shroud and, in the worst case, the tundish lining. A far more suitable solution in the long term is for the steelmakers and their suppliers to derive a site-specific solution to address the thermal loss and / or slag carry-over which leads to the crusting issues in the first place.
A full slag coverage is the principal mechanism to avoid thermal losses, as this also builds a protective barrier between the liquid steel and the atmosphere. This is shown in Fig 6a, which shows a dramatic drop in reducible oxides from 22 % (FeO + MnO) to less than 5 % combined oxide content. This work, performed in the early 1980s, underscores the significance of rapidly adding a slag at initial tundish fill for avoiding losses of iron and alloying elements. A recent trend for avoiding large reoxidation losses at initial tundish fill has been to purge the tundish with a gaseous species such as argon. which displaces the air. This normally needs the use of a physical tundish cover, and frequently the insertion of a ceramic blanket to prevent air from infiltrating the tundish environment, hence affording additional protection against contamination of the liquid steel. Improvements in this area are directed solely at the quality of cast products from the first heat on a tundish. This blanketing technique is normally confined to both the conventional and thin-section slab casting machines.
Fig 6 Changes in tundish slag chemistry and quantifying slide gate opening size
Flow control from the tundish to the mould is the last significant function performed in the tundish and can easily be considered the most critical area, since it is required to perform its functions without fail from the beginning of the tundish campaign to the last heat cast on a box. Slab casting machines use predominantly either a stopper rod system or a slide gate system for controlling flow rate from the tundish to the mould(s). Casting machines producing smaller shapes typically run with stopper rod control or metering nozzles, although a rotary combination of stopper and slide gate technology known as a PCV (precision control valve) has found some specialty applications. The PCV also has very good anti-vortexing properties.
A stopper rod is a tubular device which controls flow to the mould by adjusting the opening between the rod tip and the tundish well nozzle. A slide gate is a two-plate or three-plate system extraneous to the tundish, namely. between the tundish and the mould, which controls flow by the quantity of overlap of the circular bore openings. Stopper systems like slide gates and the PCV operate on the principle of varying the orifice opening to accommodate corrections to the casting rate needed because of the either casting speed or on-line section size (i.e., width) changes. Metering nozzles have a non-adjustable orifice and hence tend to be used on the continuous casting machine with fixed section dimensions running at constant speeds.
A stopper rod consists of a large cylindrical rounded-bottom tube which dictates flow by resulting the opening of the orifice above the tundish well nozzle (Fig 4b). Stoppers are mostly constructed of alumina-graphite, occasionally with tips incorporating the use of magnesia, ‘SiAlONs’ (pronounced ‘sy-a-lons’, an advanced class of ceramics with very good wear characteristics) and / or porous elements. As steel flows from the tundish to the mould, a negative pressure is developed at the flow control point, namely under the stopper rod tip. Different plant data show a pressure difference of around 0.17 MPa to 0.24 MPa between the tip of the stopper rod and the liquid steel above the flow control point, which in practical terms means that inert gas purging is needed to negate air aspiration through the stopper rod. Several systems do not provide purging gas to the stopper rods. These are termed ‘capped rods’. Based on fundamental principles, casting machine management is required to investigate their history of plugging for capped rods versus purged rod tips.
Stopper rods do have an advantage in terms of reducing the tendency for vortex formation, though it is a fallacy to assume that the use of a stopper eliminates the possibility of vortexing tundish slag into the mould. One additional issue to be aware of in terms of operation is that excessive erosion of the stopper tip can result in a situation where the opening between the rod and the well nozzle can no longer be sealed. This results in a so-called running stopper, and the continuous casting machines using this type of metal flow control are to incorporate procedures to cover such an eventuality.
In slide gate systems, loss of the throttling function can be overcome by several means. First, a plunger (or solid steel cone) can be jammed over the well nozzle, causing a chill-off. Alternately, the stream can be shut off in two-plate and three-plate throttling systems by moving the slide gate to a position where the orifices no longer overlap. In three-plate slide gate systems, loss of throttling plate control can normally be overcome with insertion of the so-called ‘blank’, which, as the name suggests, is a plate without a hole.
In case of the slide gate flow control, liquid steel flows into the ‘upper tundish nozzle’, through the slide gate opening, down the long SEN tube, and through the SEN ports into the mould cavity. In the 3-plate slide gate, the central plate is moved hydraulically to adjust the opening between the upper and lower stationary plates by misaligning the hole in the sliding plate relative to the nozzle bore. Alternatively, the 2-plate slide gate is missing the lowest plate, so the SEN tube is attached to the moving plate and travels as the opening is adjusted. This has the disadvantage of continuous variation in the alignment of the nozzle relative to the strand centre-line. In both systems, the joints are all flooded with low pressure inert gas (argon) to protect against air entrainment in the case of leaks.
Flow through the slide gate is governed by the size of the overlapped openings of the plates. This opening size can be quantified in several different ways. Two popular measures are ‘area opening fraction’, defined by the ratios of the opening area (shaded area) to the total bore area, and ‘linear opening fraction’, defined as the ratio of the distances S to T (Fig 6b).
The main objective of a flow regulation system is to keep and maintain the level of the liquid steel in the mould constant, i.e. avoid large and sudden mould level fluctuations. Controlling flow with a stopper rod is more difficult than with tundish slide gate because of the distance between the regulation region and the application of the control force, which is considerably longer in the case of stopper rod control system than of a tundish throttling plate. In addition, the annular opening is more sensitive to displacement. But the benefit of a stopper rod flow control is that it reduces the risks of strand freeze-up at start-up if the preheating is considered. In addition, the stopper rod delays the formation of a tundish vortex when the steel level is low. By construction, there is also less risk for air leakage possible when using a stopper rod than in the case of a multiple surfaces tundish gate system. The flow inside the casting channel which includes a stopper rod, is more uniformly distributed, so the mould flow is more symmetrical than in the case of a conventional 2-plate or 3-plate tundish slide gate.
The design of flow control components can be optimized to produce stable mould flow. This is achieved by producing a smoother flow regulation with a stopper rod and reducing flow asymmetries inside the casting channel when using a tundish slide gate.
The steel flow rate depends mainly on the height of liquid steel in the tundish driving the flow and the pressure drop across the slide gate. Flow rate naturally increases with increasing slide gate opening position and with increasing tundish depth. The flow rate is also influenced by the nozzle bore size, the quantity of gas injection, the constriction of the ports, and the extent of clogging and wear. The flow rate produced under ideal conditions can be compared with the measured flow rate in order to identify the extent of nozzle clogging.
Some tundish designs have to accommodate a considerable change in product being cast, being used on so-termed combi(nation) casting machines. These casting machines are designed to produce two different sections, such as for example a combination bloom / slab casting machine. Fig 7 shows an example of combination casting machine tundish configurations.
Fig 7 Combination casting machine tundish configurations
The external shell of the modified delta-shaped tundish is identical, with the pour box attached to the apex to the delta. However, when casting three bloom strands, the flow control devices in the tundish are set up to balance the flow between the three strands, with a V-shaped dam acting to part the flow coming from under the weir. When casting two slab strands, the centre hole is capped in the tundish set-up stage, and the V-shaped dam is replaced by two larger angled dams.
Setting of the tundish well nozzle is also a critical issue. More than a decade ago, it has been shown that misalignments of as little as 2-degree from vertical can aggravate tundish nozzle clogging, with nozzles misaligned by more than 10-degree plugging considerably faster than vertically aligned nozzles. Proper seating and grouting of the nozzles are also important for the minimization of air aspiration, especially in porous well nozzles. These types of well nozzles, typically made of magnesia or dolomite, incorporate slits for the introduction of gas to the porous refractory and are surrounded by a steel can. Since metal expands at a faster rate than the ceramic, a grout or ramming mass needs to be used for ensuring that the gas does not escape along the can instead of purging the well nozzle face as intended.
Well nozzle configuration depends a little on the casting system configuration. For example, casting machines with short sequences, e.g., less than 6 heats, typically use a one-piece well nozzle and SEN tube, which eliminates joints and prevents air aspiration. The disadvantage is, of course, that this SEN tube cannot be changed on-line, and hence a tundish-fly or continuous casting machine turnaround is needed if the system plugs to the point where casting can no longer be sustained. In systems which support SEN tube exchanges during casting, the well nozzle is linked to the so-called top plate of the tube exchange system. This is achieved either by grouting and ramming or, as more recently developed, by combining the top plate and well nozzle into a single ceramic piece, again eliminating a joint and the potential for aspiration.
The tundish design and operations can have several operational issues which the operators can face. These pitfalls to effective tundish operation can be controlled, but the shrewd supervisor of the continuous casting machine is always to be aware of their impact on the efficiency and quality of the casting process. Some of the more common operational issues in the tundish are (i) reoxidation because of the poor shrouding during teeming, (ii) reoxidation because of the turbulent slag / metal mixing in the tundish pour region, (iii) open pouring after ladle non-free-opens, combining the elements of the first two items, (iii) slag carry-over from the ladle, (iv) excessive erosion / spalling of the refractory components, (v) corrosion of the spray lining by overly aggressive tundish fluxes, (vi) non-metallic clogging in the well nozzle (oxides, nitrides and / or sulphides), (vii) freezing-type plugging in the well nozzle because of the low super-heats, (viii) skull formation because of the cold temperatures and / or poor flow distribution, and (ix) flotation of poorly anchored flow-modifying devices. This list is far from complete, but it certainly gives the beginner and expert casting operator alike reason to consider the effects of non-standard events on tundish operations, especially how cases such as those described above affect subsequent heats poured on the same box.
It is to be understood that the technologies which are extraneous to the tundish, can affect the behaviour observed in this metallurgical reactor. One example is slag sensing technology, which is used in the ladle and / or on the ladle-to-tundish transfer system. Commercially available techniques include electro-magnetic, vibration, and acoustic measurement methods which allow a distinct differentiation between the liquid steel and slag signals. There exist technologies which are evolving but not yet ‘market proven’, such as micro-wave-driven slag thickness measurements or ultra-sonic detection of vortices. Such technologies are presently being under trial and refinement, and can be ready in near future.
The first stage in quality tundish operations is the derivation of an effective design, where the term ‘effective’ is defined simply as the ‘best suited to purpose’. There is no universal design which addresses all potential casting possibilities. Rather, any particular tundish design solution is to be based on operating parameters such as product shape, number of strands, steel melting shop configuration and productivity needed, to mention only a few.
By combining the plant design concepts with operation parameters, one can derive a tundish design option which can be refined through modeling and operating experience to arrive at a final tundish design. With the physical dimensions defined, and under consideration of the refractory lining solutions to be applied, the operators and designer can now move to the task of optimizing the flow within the tundish vessel. One methodology which has been used extensively for this purpose is water modeling.
As the steel cleanliness has become a main objective for demanding steel grades the role of the tundish is nowadays both to promote inclusion removal from the liquid steel and to prevent appearance of new frequently macroscopic inclusions, which are very harmful for steel quality. Typically, these are reoxidation products formed between the ladle and the mould. Impressive developments have been done e.g., enlargement of tundish volume for longer retention time, guided flow pattern, or argon bubbling to promote inclusion removal and protection of the liquid steel against oxidation from air, slag, and refractory materials. Strong interaction between the liquid steel and tundish slag has been clearly detected. Recently, serious work to develop better covering slags for tundishes has been done and it typically involves a compromise between heat insulation and reoxidation prevention properties and capability to absorb non-metallic inclusions. Although promising results have been achieved, however, issues still appeared during transient casting conditions and long casting sequences. A growing interest in under-standing flow phenomena and inclusions behaviour in liquid steel flow, and reoxidation processes as well as inter-facial phenomena controlling inclusion transfer from steel into the slag have strongly encouraged people to address the problem again.
According to another new principle of clean steel production the function of tundish is not any more to improve steel cleanliness through inclusions removal but rather to maintain the attained level by minimizing harmful contamination of steel with air, slag, or refractory materials from ladle to moulds. This can be called a ‘protective’ or inert tundish practice. Hence, the function of tundish slag is as a barrier against reoxidation and thermal losses. By reconstructing tundish to a closed chamber with lid, argon gas shrouding and eventual heating system, tundish slag can be even ignored.
As a contrast to the ‘inert tundish practice’ a concept of ‘active tundish slag’ is here introduced. Overall, the tundish is the last reactor where important metallurgical operations can still be done. If the liquid steel interaction with the tundish slag can be optimized the liquid steel quality can be maintained and improved in the tundish. An appropriate tundish slag is able to absorb deoxidation and reoxidation products. It can also bind occasional macro-inclusions entering from ladle. Especially in transient casting conditions, start-up and ladle changes in sequential casting, the tundish conditions are most critical and the active slag can show its merits.
The most essential functions of tundish slag are summarized in Tab 1 in which also thermodynamic and kinetic constraints and means to influence interaction phenomena are given. The first item concerns a typical non-equilibrium condition between liquid steel and slag in the tundish. When the slag contains some unstable oxides, they can be reduced by some components dissolved in liquid steel. A common example is a slag containing some FeO which is then reduced by elements like aluminum, titanium, or silicon dissolved in the liquid steel. Also, other components like MnO, and even SiO2 in slag can be reduced by dissolved aluminum in aluminum-deoxidized liquid steel. These events are characterized as reoxidation reactions since they cause selective oxidation of components in liquid steel and result in formation of oxide inclusions in steel which can deteriorate cleanliness.
| Tab 1 Functions of tundish slag, thermodynamic and kinetic constraints and means to influence | ||||
| Function of slag | Thermodynamics | How to influence | Kinetics | How to influence |
| No negative interaction between slag and liquid steel | Low activity of unstable oxides FeO, MnO, and SiO2 etc. Compatibility slag / liquid steel | Low contents of unstable components | Avoid strong interaction | Avoid slag emulsification |
| Inclusion absorption capacity | Liquid, unsaturated slag capable to dissolve e.g. Al2O3 | Proper slag composition, interfacial properties | Promoting transport of inclusions to liquid steel / slag interface | Flow pattern design and control |
| Low refractory interaction | Chemical compatibility, basicity | Slag close to saturation with e.g., MgO | Flow pattern on the wall | Calm down, Marangoni flow |
| Protection against atmosphere | A uniform protecting slag barrier. Inert argon bubbling | Solid + liquid powder / slag structure | Avoid air contact with steel i.e., eye formation | Avoid too intensive flow |
| Thermal insulation | Thermal properties, thermal gradient | Solid + liquid powder / slag structure | Gradient across enough thick layer structure | Avoid too intensive flow |
Differing from the previous adverse interaction the absorption of inclusions from liquid steel to slag is a positive phenomenon. In ladles, it is one principal step in the removal stage of the deoxidation process. In a tundish the phenomenon is still possible but because of the transitory movement of the liquid steel it has much less time. Inclusions’ trajectories in the liquid steel flow through the tundish have been calculated and optimal flow patterns have been estimated to transport inclusions close to the steel / slag interface. When arrived near to the interface they have opportunity to move through the interface and dissolve into the slag.
The dissolution phenomenon is extremely important for the final steel cleanliness and, hence, has been studied rather intensively during the recent years in liquid slags in ladle / tundish / mould conditions, using different techniques such as rotating cylinder method, rotating disk, spear (sampling from the liquid slag at a specified time intervals with followed light and electron microscopy) and methods of melting the top of a steel rod together with a slag droplet.
A relatively new technique using the high temperature Confocal Laser Scanning Microscope (CLSM) has as well become available. This technique had been applied to the in-situ observation of dissolution behaviour of inclusions in liquid slags. The CLSM technique has some advantages compared with the conventional dissolution techniques. First it opens up a continuous in-situ observation of dissolution phenomena. Secondly, the ratio of dissolving species volume to solvent slag volume is very small and comparable with the situation in industrial process vessels, allowing dissolution to take place without a considerable change in the bulk composition of the slag. Thirdly, the role of interfacial tension becomes significant in the case of microscopic particles. Differences in interfacial tension between slags and steels with different compositions can have influence on inclusion removal.
The ability of a tundish slag to absorb inclusions depends on many things concerning physico- chemical properties of the system and the participating phases. Temperature of the through-flowing liquid steel determines the slag temperature at and near the interface, and together with the slag composition, the predominance area of liquid slag can be defined. Maximum solubility of ambient inclusions in liquid steel into slag can then be estimated, when mass ratios are calculated too.
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