During continuous casting of steel, liquid steel is poured from ladle to tundish where its flow is divided to each strand of the continuous casting machine. The role of the tundish is not only to act as the dispenser of the liquid steel, but also it is the last major reactor where refining process is still being carried out. The main surface where the liquid steel can be deoxidized is on the top of the tundish, where liquid steel is in contact with the atmosphere. There are several ways which prevent this reoxidation of the liquid steel such as air tight covers, establishing an argon gas rich atmosphere under the cover, and covering the surface of the liquid steel with the tundish powder etc.
Tundish powder is a free flowing non-toxic and non-hazardous mixture. It seals the surface of the liquid steel in the tundish. It is also known by several other names such as tundish covering compound, tundish covering flux, tundish flux, tundish covering slag, or tundish flux slag. Tundish powder has got very good insulating properties. It plays an important role in the continuous casting of liquid steel and is one of the influential and critical factors in the stable operation of the continuous casting machine. The use of a suitable tundish powder is an essential feature of the present day tundish metallurgical practice. However, the selection of powder is specific to the grade of steel and product quality requirements.
With the development of the high speed casting, tundish powder is becoming the key material for the continuous casting process and has more and more important and irreplaceable roles on the quality of the cast steel. The main functions of the tundish powder are (i) to protect liquid steel against oxidation, (ii) to control, optimize, and insulate the heat transfer from the liquid steel to the ambient and thus prevent the drop in the temperature of the liquid steel, (iii) to absorb the non-metallic inclusions from the liquid steel to produce cleaner cast steel product, (iv) to provide chemical protection to the liquid steel from oxidation and other undesired reactions, and (v) to be compatible with the refractory materials which are used as the containment materials in the tundish.
The above functions of the tundish are contradicting requirements in many respects. These requirements necessitate (i) that a layer of liquid slag be in contact with the liquid steel, (ii) that the composition of this liquid slag layer be closely controlled so as to effectively absorb inclusions, and (iii) that a solid but porous layer covers the liquid slag to ensure effective thermal insulation.
The continuous casting tundish serves as an interim reservoir when liquid steel is transferred from the steel ladle to the continuous casting mould. The tundish can also serve as a metallurgical refining vessel (or reactor) since it presents several opportunities for the effective treatment of liquid steel. Over the past decades, in the design and assessment of tundish metallurgy, the optimization of tundish powder properties is increasingly being recognized as a major contribution towards improving steel cleanliness.
Tundish powders are basically of two types. The first type is acidic in nature and is normally based on rice husk ash or fly ash. The second type is basic in nature and normally is based on MgO (magnesium oxide). Today, several types of tundish powders with different compositions and shapes (granular, powder form, and extruded powders) are produced to suit the casting of diverse steel grades. The factors which influence the properties of tundish powders are (i) chemical composition, (ii) mineralogical composition, (iii) grain size composition, (iv) manufacturing process, (v) drying or roasting method, and (v) free carbon content. Each shape and type of the tundish powder has its own advantages and disadvantages, such as price, health issues, flow-ability, thermal insulation, and melting rate.
Traditionally tundish powder is made from rice husk or fly ash which contain high percentage of SiO2 (silica) and provides very good protection from heat loss. These materials form high viscosity liquid slag which is unable to successfully absorb inclusions. Also high SiO2 content is a source of reoxidation. That is why, these days, custom made tundish powders which form complex phases are used. SiO2 based tundish powders are acidic powders. With the increasing demands upon the performance of tundish powder, steelmakers are switching over from acidic (or silica based materials) powders to basic (or calcium-based) powders.
The main purpose of the tundish powder is to reduce heat loss through radiation and forming a barrier between atmosphere and the liquid steel. Recent developments have been done to improve capabilities of the tundish powder to absorb inclusions which arrive to the phase boundary between the liquid steel and the tundish powder.
Besides insulating the surface of the liquid steel, the tundish powder is designed (i) to be cost effective, (ii) to have high expansion properties, (iii) to prevent skull formation of steel in the tundish walls, (iv) to increase the holding time of the liquid steel in the tundish, (v) to have low consumption rate (in the range of around 0.8 kg/ton to 1.2 kg per ton, (vi) to have high saving of energy by limiting of the heat-loss, (vii) to be inert to the tundish refractory, (viii) to have mineral structure which is highly absorbent and which cleans the slag and residuals in the tundish, and thus increase the lifetime of the refractory bricks, (ix) to improve the working conditions for the workmen by preventing burning of the insulation mixture when it is in contact with the liquid steel, and (x) to withstand service temperatures of upto 1,750 deg C.
Tundish powder is normally engineered for use on a wide range of steel grades and casting facilities ranging from billet, bloom, and traditional and thin slab casting machines. It is required to have very good expandability and spreadability, so that it expands and spreads instantly and homogenously cover entire liquid steel surface in the tundish, creating a liquid slag layer. It is to keep the slag liquid so that the same can be easily poured in the slag ladle, after the casting. It has to have low bulk density and is to be a free flowing non-toxic and non-hazardous mixture. Due to its content based on the natural minerals, it seals the surface of the liquid steel in the tundish. If the tundish powder is not based on rice husk and fly ash then it avoids the major problem which is related to the concerns related to the working environment. In case of the tundish powder based on rice husk and fly ash, the powder because of its low bulk density and small particle size, some of the particles become airborne when it comes in contact with the liquid steel. This happens since there is no thermal conversion of SiO2 into crystalline cristobalite which make it non health hazard.
During the sintering process, tundish powder starts to melt and forms liquid phase. Composition of tundish powder is required to be just right, so that it forms a liquid slag layer on the contact with the liquid steel, and is still solid above this layer. The liquid phase protects steel from reoxidation and the solid phase acts as a thermal insulator. Fig 1 shows schematic diagram of three components of tundish powder in a continuous casting tundish.
Fig 1 Schematic diagram of three components of tundish powder in a continuous casting tundish
These days, the tundish is increasingly being used as a refining reactor. In the earlier days, tundish powder was mainly used for the thermal insulation. During this period, SiO2 based tundish powder has extensively been used. Use of SiO2 based tundish powder was mostly driven by the economic considerations. These SiO2 based tundish powders (acid compositions) are not effective in preventing reoxidation of the steel and cannot absorb large quantities of Al2O3 (alumina) inclusions. Moreover, the use of an acidic tundish powder in the tundish causes re-sulphurization from the carry-over ladle slag. Also, SiO2 in the tundish slag is reduced by aluminum dissolved in the steel, with the result that oxide cleanliness is adversely affected through the formation of Al2O3. The formation of Al2O3 by this mechanism can to some extent be counter-acted by an increase in the silicon activity in the steel (a high silicon content in the steel) and / or a decrease in the aluminum activity (a low aluminum content in the steel). However, since the silicon and aluminum contents in the steel are defined by the specific steel composition which is to be produced, these are not independent variables and hence, a reduction of the SiO2 content in the slag remains the only alternative approach.
A variety of materials such as calcium aluminate based ladle fluxes, mould powders and phosphorus furnace slags have been used as tundish powder in industrial practice. However, none of these materials meet the demands of an effective tundish powder and a better approach is to attempt to design a tundish powder from first principles by using locally available materials. The use of locally available raw materials has the added advantage in that the cost of the tundish powder can be reduced. This approach needs that the requirements, which the tundish powders are to meet, are well understood.
Since heat transfer through a completely liquid layer is rapid, tundish flux needs an outer solid particulate layer. A layer of liquid in contact with the liquid steel is necessary to protect the liquid steel from the atmospheric oxidation and also to absorb inclusions which reach the powder-metal interface. In addition, the melting temperature and melting rate of the tundish powder are critically important properties to balance the requirement to have a layer of liquid slag in contact with the liquid steel and a porous solid layer in contact with the atmosphere. In this respect, it is important to note that the melting rate increases as the bulk density of the powder decreases and also with a decrease in carbon content of the powder. The melting rate also increases as the carbonate content and thermal conductivity of the tundish powder are increased.
The melting rate, crystallization temperature, capacity to absorb inclusions, fluidity, and compatibility with refractory materials, determine the effectiveness of the tundish powder. There are certain properties of the tundish powder which are required to be measured to give an indication of powder performance. Earlier studies in this respect have shown that the melting temperature, viscosity, density, thermal conductivity, basicity (V=CaO/SiO2) and CaO/Al2O3 ratio affect the behaviour of the tundish powder. One of the issues is that in some cases an improvement of one measurable indicator can reduce another. For an example, low basicity reduces the ability of the powder to absorb inclusions but increases fluidity and a tundish powder with a low melting point effectively prevents oxidation and increases the ability of the tundish powder to absorb inclusions but it leads to considerable higher heat losses. Hence, a balance as to be struck between the various requirements and powder performance need to be optimized while designing a suitable tundish powder. This is to be done not only from a technical point of view but also economic considerations such as cost, availability, cost of transportation, and mixing and pre-melting costs need to be taken into consideration.
Tundish powder is required to form a liquid slag layer which in turn is required to cover the surface of the liquid steel quickly and effectively. Studies have shown that the decomposition of carbonates, as measured by the loss on ignition (LOI) provide a gas layer at the interface of the liquid steel and tundish powder layer. This gas layer acts as a low friction surface thereby increasing the ability of the material to flow so as to quickly and easily spread over the liquid steel and to reach the corners of the tundish. Loss of Ignition values are normally kept below 10 %. Tab 1 gives typical compositions of some tundish powders.
|Tab 1 Typical chemical composition of tundish powders|
|Tundish powder||CaO||MgO||Al2O3||SiO2||FeO||MnO||Na2O+K2O||Total carbon||LOI|
|Note: Tundish powders A to G are produced directly from the raw materials without pre-melting. Tundish powders A1 to H1 contains pre-fused materials.|
In the continuous casting steelmaking process, along with the casting process, the liquid slag layer is constantly consumed by the liquid steel and gets supplemented from the sintered layer. Similarly, the sintered layer is constantly transformed into liquid slag layer and gets supplemented from the powder layer. The operator at the tundish keeps an eye on the powder layer on a continuous basis and adds new tundish powder into the tundish as necessary.
The melting point of the tundish powder (around 1260 deg C) is lower than the temperature of the liquid steel (around 1560 deg C). The tundish powder in a tundish is divided into three main parts as per the physical form, namely, liquid slag layer, sintered layer, and powder slag layer as shown in Fig 2. Tundish powder is added on the top of the liquid steel in the tundish, which partially melts and forms a liquid slag layer (around 6 mm to 15 mm thick) above the liquid steel. The liquid slag layer can partly prevent the heat of liquid steel from transferring and hence, above the liquid slag layer, temperature drops, which forms a sintered layer (temperature around 600 deg C to 1,080 deg C). Above the sintered layer, more heat is prevented and hence, the tundish powder can keep its original pulverous characteristic to form a powder slag layer. The powder slag layer covers the liquid steel surface evenly to prevent the heat radiation and isolate oxygen. Fig 2 shows cross-section through a typical layer of tundish powder and slag in a continuous casting tundish.
Fig 2 Cross-section through a typical layer of tundish powder and slag in a tundish
Liquid steel in tundish comes in contact with liquid powder slag and lining refractory, which are based on Al2O3 and SiO2 or MgO, respectively. The composition of the tundish powder is to be such that it does not react with tundish lining. Further, liquid steel and the tundish powder have different conductivities and hence, these two materials have different temperatures, and there is obvious temperature gradient in the interface layer of these two materials.
While it is relatively easy to select a suitable tundish powder on the basis of their MgO (magnesium oxide) content, the position with respect to Al2O3 content is distinctly more complex. A low percentage of Al2O3 increases the alumina absorption capacity of the tundish powder but, at the same time, it also increases the CaO / Al2O3 ratio which, in turn, leads to high melting and crystallization temperatures of the tundish powder and therefore to the formation of solid phase particles in the liquid slag zone at the tundish operating temperatures. The alumina content of some of the tundish powders in Tab 1 is very low and their potential use is to be done with caution.
It is reasonably well established that the CaO / SiO2 ratio in an effective tundish powder is to be higher than 2. On the other hand, too high basicity leads to undesirable low viscosities and to an unacceptable decrease in the crystallization temperature and a high FeO content is undesirable since the iron oxide is not stable at the tundish temperature and reacts with alloying elements in the steel to form oxide inclusions.
Several studies have shown that the preferred bulk density of tundish powders lies in the range 400 kg/cum to 800 kg/cum although tundish powder densities as high as 1,100 kg/cum have been used. The melting rate of tundish powder is to be sufficiently high to ensure that a liquid layer forms on the surface of the liquid steel immediately after the introduction of the powder into the tundish, to prevent oxidation of the steel but too high a melting rate increases the heat losses to the atmosphere. Normally, a melting rate of less than 0.7 mm / second is considered to be optimal.
The thermal conductivity of the tundish powder is to be less than 1 W/mK since higher thermal conductivity leads to excessive heat loss through the surface. The viscosity of the tundish powder is to be less than 0.3 Pa.s. Tundish powder with a viscosity higher than this value causes problems with the fluidity of the liquid slag and a viscosity lower than 0.3 Pa.s is also required to ensure efficient inclusion absorption. A low viscosity and a high basicity means that the length of the SiO2 chains in the molten structure are rather short. Accordingly, ion mobility and short-range order, both being pre-requisites for efficient crystallite nucleation, are more readily attained. Very viscous or low basicity tundish powder, on the other hand, is indicative of extended SiO2 ion chains. The addition of Al2O3 to a low basicity tundish powder suppresses crystal precipitation and serves as a glass former due to chain formation role in oxide melt structures.
Tundish powder has substantial percentage of Al2O3 and SiO2 as well as aluminum and carbon. After tundish powder is added to the liquid steel, processes like sintering of powder and oxidizing of aluminum and carbon start. After aluminum and carbon oxidize main component in the tundish powder are alumino-silicates. During sequence casting, the composition of the tundish powder slag changes dramatically. Main components of the tundish powder slag at the end of casting are gehlenite (2CaO.Al2O3.SiO2) and calcium aluminate (CaO.2Al2O3). Also spinel [(MgO, MnO).Al2O3] which can have originated from refining process in ladle can be found in it.
In the tundish, during the casting process, there is a shift in the chemical composition of the tundish powder from binary system of Al2O3-SiO2 based mullite (3Al2O3.SiO2) over spessartite (3MnO.Al2O3.3SiO2) towards gehlenite (2CaO.Al2O3.SiO2) and calcium aluminates. Influx of CaO in such great quantities can come through ladle slag carryover. The most important change to tundish powder slag composition comes from ladle slag carryover due to vortex effect.
In penetrated zone of contact lining with the tundish powder slag are phases which have transformed from original periclase (MgO) towards enstatite (MgO.SiO2), monticellite (CaO.MgO.SiO2) and merwinite (3CaO.MgO.2SiO2). Spinel (MgO.Al2O3), if found in the tundish powder slag, originates from ladle slag or from a reaction between Al2O3 in cover slag and MgO from tundish lining.
Formation of CaO containing compounds shows that some ladle slag carry over has occurred. If chromite is present in the tundish powder slag, then is has its origin from the sand of the ladle well block. In the boundary zone between tundish lining, liquid steel, and the tundish powder slag, it can come to penetration of the latter. Magnesia based lining transforms to merwinite (3CaO.MgO.2SiO2). Depth of penetration can range from 2 mm to 3 mm in a sequence of two heats. Deposits from submerged entry nozzle are mainly composed from calcium aluminate (CaO.2Al2O3) and spinel [(MgO, MnO).Al2O3). All these informations help in the optimization of the tundish powder slag composition which in turn helps in entrapping more inclusions while sufficiently protecting the steel from heat loss and reoxidation.