Basics of Continuous Casting of Steel
Basics of Continuous Casting of Steel
Continuous casting is a process whereby liquid steel is solidified into a semi-finished product. Cross-sections can be rectangular (slab, thin slab, or bloom) for subsequent rolling into plate, sheet, or heavy sections, square (billet or bloom) for rolling into long products, circular (round) for rolling into seamless pipes or tubes, and even ‘dog-bone’ shapes for rolling into ‘I’ or ‘H’ beams. Billets have cross section with sizes up to about 200 mm square. Blooms have cross section either square or rectangular with size of each side ranging from greater than 200 mm to less than 600 mm. Rounds include diameters of 125 mm to 500 mm. Beam blanks are shaped like dog bones, and slabs are cast with size range of thickness from 200 mm to 400 mm, and width range from greater than 500 mm to 2,500 mm. The width to thickness ratio, referred to as the ‘aspect ratio’, is used to determine the dividing line between blooms and slabs. An aspect ratio of 2.5:1 or greater constitutes an as-cast product referred to as a slab. Thin slab has thickness in the range of 50 mm to 90 mm.
A wide range of steel grades ranging from ultra-low carbon and low carbon grades to high carbon and different grades of special steels are required to be cast in the continuous casting machine. The casting of these grades is to be achieved while maximizing the output of the continuous casting machine. Consistent production of prime quality cast steel product needs increased operational and maintenance flexibility in the continuous casting machine so that optimum casting parameters can be maintained. This flexibility is needed both for every element as well as control system of the continuous casting machine.
Continuous casting transforms liquid steel into solid on a continuous basis. The process is the most efficient way to solidify large volumes of liquid steel into simple shapes for subsequent processing. Continuous casting is distinguished from other solidification processes by its steady state nature. The liquid steel solidifies against the mould walls while it is simultaneously withdrawn from the bottom of the mould at a rate which maintains the solid / liquid interface at a constant position with time. The process works best when all of its aspects operate in this steady-state manner. Relative to other casting processes, continuous casting process normally has a higher capital cost, but lower operating cost. It is the most cost- efficient and energy- efficient method to mass-produce semi-finished steel products with consistent quality in a variety of sizes and shapes.
It is the most frequently used process to cast liquid steel. Since its widespread introduction for steel in the 1950s, it has evolved to achieve improved yield, quality, productivity, and cost efficiency. It allows lower-cost production of steel sections with better quality, because of the inherently lower costs of continuous, standardized production of a product, as well as providing increased control over the process through automation. However, challenges remain and new ones appear, as methods are sought to minimize casting defects.
Continuous casting of steel is a relatively new process in historical terms. Although the continuous strip casting process was conceived by Henry Bessemer in 1858 and patented in 1865, the continuous casting of steel did not gain widespread use until the 1960s. Earlier attempts suffered from technical difficulties such as ‘breakouts’, where the solidifying steel shell sticks to the mould, tears, and allows liquid steel to pour out over the bottom of the machine. This problem was overcome by Junghans in 1934 by vertically oscillating the mould, utilizing the concept of ‘negative strip’ where the mould travels downward faster than the steel shell during some portion of the oscillation cycle to dislodge any sticking. Several other developments and innovations have transformed the continuous casting process into the sophisticated process presently used to produce around 97 % of the crude steel in the world today, including plain carbon, alloy, and stainless-steel grades.
Continuous casting has replaced several steps during steel making process such as ingot casting, mould stripping, heating in soaking pits, and primary rolling with one operation. Continuous casting of steel has helped to achieve improved yield, quality, productivity and cost efficiency. The principle of continuous casting is shown in Fig. 1.
Fig 1 Principle of continuous casting
Referring to Fig 1, liquid steel in the steel teeming ladle (1) from the secondary steel making unit is taken to the continuous casting machine. The ladle is raised onto a turret which rotates the ladle into the casting position above the tundish (3). Liquid steel flows out of the ladle into the tundish, and then into a water-cooled copper mould (5). Solidification begins in the mould, and continues through the roll support (6) and the turning zone (7). The continuous cast strand is then straightened, torch-cut, and discharged for intermediate storage or hot charged for finished rolling.
Depending on the product end use, various shapes are cast. In conventional continuous casting machines, these are slabs, blooms or billets. In recent years, the melting, casting, and rolling processes have been linked while casting a shape which substantially conforms to the finished product. These near net shape cast sections are normally applied to beams and flat rolled products, and results in a highly efficient operation. In the case of near net shape casting, the complete process chain from liquid steel to finished rolling can be achieved within two hours.
Several different types of continuous casting processes exist. Fig 2 shows a few of the most important ones. Vertical continuous casting machines are used to cast aluminum and a few other metals for special applications. Curved machines are used for the majority of steel casting and needed bending and / or unbending of the solidifying strand. Horizontal casting features a shorter building and is used occasionally for both nonferrous alloys and steel. Finally, thin strip casting is being pioneered for steel and other metals in low-production markets in order to minimize the quantity of rolling needed.
Fig 2 Major types of continuous casting machine
In the design of continuous casting machines, the factors which are important to be considered are (i) end use product influences the quality, grade and shape of the cast product, (ii) annual tonnage to be cast, (iii) availability of liquid steel and heat size, and (iv) planned operating hours. These factors dictate the continuous machine design parameters such as the number of cast strands and casting speed which is needed to match the liquid steel supply to the continuous casting machine. Quality and grade of the steel to be cast are utilized in determining various design parameters of the casting machine such as its length, vertical height, curved or straight mould, water versus air mist cooling, and electromagnetic stirring etc.
There are two steps which are involved for the transfer of liquid steel from the steel teeming ladle to the mould of continuous casting machine. These are (i) transferring or teeming of liquid steel from the teeming ladle to the tundish and (ii) transfer of liquid steel from the tundish to the moulds. Regulation of liquid steel flow from tundish to mould occurs through orifice devices of different designs such as slide gates, stopper rods, or metering nozzles, the latter controlled by tundish steel level adjustment.
Continuous casting process
For starting the casting of a fresh heat, the bottom of the mould is sealed by a steel dummy bar, which is held in place hydraulically by the straightening withdrawing unit. This dummy bar prevents liquid steel from flowing out of the mould. The liquid steel poured into the mould is partially solidified, producing a steel strand with a solid outer shell and a liquid core. In this primary cooling area, once the steel shell has a sufficient thickness, around 10 mm to 20 mm, the straightening withdrawal unit is started and proceeded to withdraw the partially solidified strand out of the mould along with the dummy bar. Liquid steel continues to pour into the mould to replenish the withdrawn steel at an equal rate. The withdrawal rate depends on the cross-section, grade, and quality of steel being produced, and can vary between 300 mm and 7,500 mm per minute.
Casting time is typically 45 minutes to 90 minutes per heat to avoid excessive ladle heat losses. After leaving the mould, the cast steel strand enters a roller containment section and secondary cooling chamber in which the solidifying strand is sprayed with water, or a combination of water and air (air mist) to promote solidification. This area preserves cast shape integrity and product quality. Larger cross section needs extended roller containment. Once the strand is fully solidified and has passed through the straightening-withdrawal units, the dummy bar is disconnected, removed and stored. Following the straightening, the strand is cut into individual pieces of the as cast products (slabs, blooms, billets, rounds, or beam blanks depending on the machine design).
The schematics of the continuous casting process for steel is shown in Fig 3. In this process, liquid steel flows from a ladle through a tundish into the mould. The tundish holds enough liquid steel to provide a continuous flow to the mould, even during an exchange of ladles, which are supplied periodically from the secondary steelmaking process. The tundish can also serve as a refining vessel to float out detrimental inclusions into the slag layer. If solid inclusion particles are allowed to remain in the product, then surface defects such as ‘sliver’ can form during subsequent rolling operations, or they can cause local internal stress concentration, which lowers the fatigue life. To produce higher quality product, the liquid steel is to be protected from exposure to air by a slag cover over the liquid surface in each vessel and by using ceramic nozzles between vessels. If not, then oxygen in the air reacts to form detrimental oxide inclusions in the steel.
Fig 3 Schematics for continuous casting of steel
Once in the mould, the liquid steel freezes against the water-cooled walls of a bottomless copper mould to form a solid shell. The mould is oscillated vertically in order to discourage sticking of the shell to the mould walls. Drive rolls lower in the machine continuously withdraw the shell from the mould at a rate or ‘casting speed’ which matches the flow of incoming liquid steel, so the process ideally runs in steady state. Below mould exit, the solidifying steel shell acts as a container to support the remaining liquid. Rolls support the steel to minimize bulging due to the ferrostatic pressure.
The liquid flow rate is controlled by restricting the opening in the nozzle according to the signal fed back from a level sensor in the mould. The most critical part of the process is the initial solidification at the meniscus, found at the junction where the top of the shell meets the mould, and the liquid surface. This is where the surface of the final product is created, and defects such as surface cracks can form, if problems such as level fluctuations occur. To avoid this, oil or mould slag (casting powder) is added to the steel meniscus, which flows into the gap between the mould and shell. In addition to lubricating the contact, a casting powder layer protects the steel from air, provides thermal insulation, and absorbs inclusions.
Below the mould exit, the thin solidified shell acts as a container to support the remaining liquid, which makes up the interior of the strand. Water or air mist sprays cool the surface of the strand between the support rolls. The spray flow rates are adjusted to control the strand surface temperature with minimal reheating until the liquid core is solid. After the center is completely solid at the ‘metallurgical length’ of the continuous casting machine, which is 10 m to 40 m, the strand is cut either with oxy-fuel torches or with mechanical shears into cast product of any desired length.
Different continuous casting processes exist to produce cross sections of different shapes and sizes. Heavy, four-piece plate moulds with rigid backing plates are used to cast large, rectangular ‘slabs’, (50 mm to 300 mm thick and 0.5 m to 2.2 m wide), which are rolled into plate or sheet. Similar moulds are used for casting relatively square ‘blooms’, which range up to 400 mm x 600 mm in cross section. Single-piece tube molds are used to cast small, square ‘billets’ (up to a range of 100 mm to 200 mm thick) which are rolled into long products, such as bars, angles, squares, and channels etc.
The new strip casting process is being developed using large rotating rolls as the mould walls to solidify 1 mm to 3 mm thick steel sheet. When casting large cross sections, such as slabs, a series of rolls are necessary to support the soft steel shell between mould exit and the metallurgical length, in order to minimize bulging because of the internal liquid pressure. Extra rolls are needed to force the strand to ‘unbend’ through the transition from the curved to the straight portion of the path. If the roll support and alignment are not sufficient, internal cracks and segregation can result. These defects persist in the final product, even after several rolling and other operations, so it is important to control the casting process.
The continuous casting process is started by plugging the bottom of the mould with a ‘dummy bar’. After enough liquid steel has solidified like a conventional casting onto its head, the dummy bar is then slowly withdrawn down through the continuous casting machine and steady state conditions evolve. The process then operates continuously for a period of one hour to several weeks, when the liquid steel supply is stopped and the process is to be restarted. The maximum casting speed of 1 metre per minute (m/min) to 8 m/min is governed by the allowable length of the liquid core, and to avoid quality problems, which are normally worse at higher speeds.
Some of the important phenomena which govern the continuous casting process and determine the quality of the product are shown in Fig 4. Liquid steel flows into the mould through ports in the submerged entry nozzle, which is normally bifurcated. The high velocities produce Reynolds numbers exceeding 100,000 and fully-turbulent behaviour. Argon gas is injected into the nozzle to prevent clogging. The resulting bubbles provide buoyancy which affects greatly the flow pattern, both in the nozzle and in the mould. The bubbles also collect inclusions which can become entrapped in the solidifying shell, leading to serious surface defects in the final product.
Fig 4 Important phenomena of continuous casting process
The jet leaving the nozzle flows across the mould and impinges against the shell solidifying at the narrow face. The jet carries superheat, which can erode the shell where it impinges on locally thin regions. In the extreme, this can cause a costly breakout, where liquid steel bursts through the shell. Typically, the jet impinging on the narrow face splits to flow upwards towards the top free surface and downwards toward the interior of the strand.
Flow recirculation zones are created above and below each jet. This flow pattern changes radically with increasing argon injection rate or with the application of electromagnetic forces, which can either brake or stir the liquid. The flow pattern can fluctuate with time, leading to defects, so transient behaviour is important. Liquid flow along the top free surface of the mould is very important to steel quality. The horizontal velocity along the interface induces flow and controls heat transfer in the liquid and solid flux layers, which float on the top free surface. Inadequate liquid flux coverage leads to nonuniform initial solidification and a variety of surface defects.
If the horizontal surface velocity is very high, the shear flow and possible accompanying vortices can entrain liquid flux into the steel. This phenomenon depends largely on the composition dependent surface tension of the interface and possible presence of gas bubbles, which collect at the interface and can even create a foam. The flux globules then circulate with the steel flow and can later be entrapped into the solidifying shell lower in the caster to form internal solid inclusions. The vertical momentum of the steel jet lifts up the interface where it impinges the top free surface. This typically raises the narrow face meniscus, and creates a variation in interface level, or ‘standing wave’, across the mould width. The liquid flux layer tends to become thinner at the high points, with detrimental consequences.
Transient fluctuations in the flow cause time-variations in the interface level which lead to surface defects such as entrapped mould powder. These level fluctuations can be caused by random turbulent motion, or changes in operating conditions, such as the sudden release of a nozzle clog or large gas bubbles. The liquid steel contains solid inclusions, such as alumina. These particles have different shapes and sizes and move through the flow field while colliding to form larger clusters and can attach to bubbles. They either circulate up into the mould flux at the top surface, or are entrapped in the solidifying shell to form embrittling internal defects in the final product.
Casting powder is added to the top surface to provide thermal and chemical insulation for the liquid steel. This oxide-based powder sinters and melts into the top liquid layer which floats on the top free interface of the steel. The melting rate of the powder and the ability of the molten flux to flow and to absorb detrimental alumina inclusions from the steel depends on its composition, governed by time-dependent thermodynamics. Some liquid flux resolidifies against the cold mould wall, creating a solid flux rim which inhibits heat transfer at the meniscus. Other flux is consumed into the gap between the shell and mould by the downward motion of the steel shell, where it encourages uniform heat transfer and helps to prevent sticking.
Periodic oscillation of the mould is needed to prevent sticking of the solidifying shell to the mould walls, and to encourage uniform infiltration of the mould flux into the gap. This oscillation affects the level fluctuations and associated defects. It also creates periodic depressions in the shell surface, called ‘oscillation marks’, which affect heat transfer and act as initiation sites for cracks. Initial solidification occurs at the meniscus and is responsible for the surface quality of the final product. It depends on the time-dependent shape of the meniscus, liquid flux infiltration into the gap, local superheat contained in the flowing steel, conduction of heat through the mould, liquid mould flux and resolidified flux rim, and latent heat evolution.
Heat flow is complicated by thermal stresses which bend the shell to create contact resistance, and nucleation undercooling, which accompanies the rapid solidification and controls the initial microstructure. Further solidification is governed mainly by conduction and radiation across the interfacial gap between the solidifying steel shell and the mould. This gap consists mainly of mould flux layers, which move down the mould at different speeds. It is greatly affected by contact resistances, which depend on the flux properties and shrinkage and bending of the steel shell, which can create an air gap. The gap size is controlled by the quantity of taper of the mould walls, which is altered by thermal distortion. In addition to controlling shell growth, these phenomena are important to crack formation in the mould because of thermal stress and mould friction, which increases below the point where the flux becomes totally solid.
As solidification progresses, micro-segregation of alloying elements occurs between the dendrites as they grow outward to form columnar grains. The rejected solute lowers the local solidification temperature, leaving a thin layer of liquid steel along the grain boundaries, which can later form embrittling precipitates. When liquid feeding cannot compensate for the shrinkage because of solidification, thermal contraction, phase transformations, and mechanical forces, then tensile stresses are generated. When the tensile stresses concentrated on the liquid films are high enough to nucleate an interface from the dissolved gases, then a crack forms.
After the shell exits the mould and moves between successive rolls in the spray zones, it is subject to large surface temperature fluctuations, which cause phase transformations and other microstructural changes which affect its strength and ductility. It also experiences thermal strain and mechanical forces because of the ferrostatic pressure, withdrawal, friction against rolls, bending, and unbending. These lead to complex internal stress profiles which cause creep and deformation of the shell. This can lead to further depressions on the strand surface, crack formation, and propagation.
Lower in the caster, fluid flow is driven by thermal and solutal buoyancy effects, caused by density differences between the different compositions created by the micro-segregation. This flow leads to macro-segregation and associated defects, such as centerline porosity, cracks, and undesired property variations. Macro-segregation is complicated by the nucleation of relatively pure crystals, which move in the melt and form equiaxed grains which collect near the centerline. Large composition differences through the thickness and along the length of the final product can also arise because of the intermixing after a change in steel grade. This is governed by transient mass transport in the tundish and liquid portion of the strand.
Sections of a continuous casting machine
The continuous casting machine consists several sections as described below.
Tundish – 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.
The shape of the tundish is typically rectangular, but delta and ‘T’ shapes are also sometimes used. Nozzles are located along its bottom for the distribution of liquid steel to the moulds. The tundish also serves several other key functions which include (i) enhances oxide inclusion separation, (ii) provides a continuous flow of liquid steel to the mould during the exchange of ladles, (iii) maintains a steady liquid steel height above the nozzles to the moulds, thereby keeping steel flow constant and hence casting speed constant as well (for an open-pouring metering system), and (iv) provides more stable stream patterns to the moulds.
Tundish smoothens out flow, regulates steel feed to the mould and cleans the liquid steel. 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 hence 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.
Water cooled copper mould – Moulds play an important role in the process of continuous casting of liquid steel. They are the heart of the continuous casting process. In the process of continuous casting, liquid steel is poured from the tundish into the casting mould through the submerged entry nozzle (SEN) immersed in the liquid steel. The moulds are water cooled. Solidification of liquid begins in the mould by indirect cooling. The cooling process in the mould is known as primary cooling process.
The mould is basically an open-ended box structure, containing a water-cooled inner lining fabricated from a high purity copper alloy. Small quantities of alloying elements are added to increase the strength. Mould water transfers heat from the solidifying shell. The working surface of the copper face is frequently plated with chromium or nickel to provide a harder working surface, and to avoid copper pickup on the surface of the cast strand, which can facilitate surface cracks on the cast steel. The depth of the mould can range from 0.5 m to 2 m depending on the casting speed and section size.
Mould is tapered to reduce the air gap formation. Taper is typically 1 % of the mould length. For cross-section of mould, the taper is around 1 mm for 1 m long mould. The cross section of the mould is the cross section of the section being cast. Mould cross section decreases gradually from top to bottom.
Once the liquid steel refining process is completed during steelmaking, the liquid steel contained in the ladle is normally sent to a continuous casting machine. The steel is poured from the ladle to a tundish and then from the tundish into a water-cooled copper mould which induces the formation of a thin, solidified steel shell (Fig 5). Flow between vessels is driven by gravity. Between the tundish and the 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 metal level in the mould, which is known as ‘meniscus’, is very important for the surface quality of the cast product. Accurately controlled and consistent conditions are needed for all parameters in the mould for the surface quality of the cast product.
Fig 5 Schematic of mould region of continuous process for steel slabs
The function of continuous casting mould is to receive the liquid steel and guarantee a rapid heat transfer to the cooling water to enable quick solidification. The liquid steel, when leaving the mould, is to have a just thick enough outer shell to prevent it from splashing over the continuous casting machine parts. The mould is to serve this function. After the mould, further cooling of the steel strand is done through the faster direct cooling with the help of the direct water sprays.
The main purpose of the mould is to produce and stabilize a solid shell resistant enough to contrast the metallic pressure of the liquid core and, hence, contain the liquid phase at the entry of the secondary spray cooling zone. If the mould system does not work properly, a break-out can take place and the hot li`quid steel core can burst open, pouring liquid steel onto the machine and causing a very dangerous situation.
Solidification arises from the dynamic nature of the casting process. In particular this relates to (i) handling of very high heat flux in the mould, (ii) nurturing of the initial thin and fragile solid shell for avoidance of breakout during descent of the strand down the mould, and (iii) designing of casting parameters in tune with the solidification dynamics of the steel grade for minimization or elimination of surface and internal defects in the cast product
Key elements of the steel shell which is leaving the mould are shape, shell thickness, uniform shell temperature distribution, defect free internal and surface quality with minimal porosity, and few non- metallic inclusions.
Mould oscillation – Mould oscillation is necessary to minimize friction and sticking of the solidifying shell, avoidance of shell tearing, and liquid steel breakouts. Breakouts can cause major damage to equipment and a large machine downtime is needed because of clean up and repairs. Friction between the shell and mould is reduced through the use of mould lubricants such as oils or powdered fluxes. Oscillation is achieved either hydraulically or through motor driven cams or levers which support and reciprocate (or oscillate) the mould. Mould oscillating cycles vary in frequency, stroke, and pattern. However, a common approach is to employ what is called ‘negative strip’, a stroke pattern in which the downward stroke of the cycle enables the mould to move down faster than the section withdrawal speed. This enables compressive stresses to develop in the shell which increase its strength by sealing surface fissures and porosity.
In addition to the heat extraction, mould oscillation is fundamental to the continuous casting. Mould shell friction is required to be minimized in order to eliminate steel sticking, tearing, and cracking. Oscillators are simple machines which reciprocate the billet mould to help prevent the steel from sticking to the mould wall. The mould is normally oscillated in a sinusoidal mode, with typical stroke and oscillation frequency parameters being 10 mm and 2 hertz (Hz) respectively. Mould oscillation parameters for minimizing of sticking and the oscillation mark depth are stroke and negative strip time. Negative strip time is defined as the time period during which the mould moves faster downward than the strand withdrawal rate. Mould lead is the distance the mould moves past the shell during the negative strip. For billet casting, the recommended mould lead and the negative strip time values are 3 mm to 4 mm and 0.12 seconds to 0.15 seconds respectively. Continuous casting machines with negative strip times below 0.1 seconds and mould lead below 2 mm to 3 mm is susceptible to mould shell sticking especially if the meniscus is fluctuating. Mould leads higher than 5 mm can contribute to deeper, non-uniform oscillation marks.
The surface of the continuously cast product is characterized by the presence of oscillation marks which form periodically at the meniscus because of the mould reciprocation. Each oscillation mark is a local depression of the steel and hence causes an increase locally in the width of the steel / mould gap. Hence, heat removal is locally reduced in the vicinity of the oscillation marks. Depending on the depth of the oscillation marks, locally reduced shell thickness, breakouts or transverse surface cracks can appear. The pitch of the oscillation marks on the surface of the strand is linked to the frequency of the oscillation cycle.
Electro-magnetic stirring – Electro-magnetic stirring is a direct and powerful technique for controlling the solidification process in the continuous casting of liquid steel. A significant, but not the only advantage of the electro-magnetic stirring, is improved quality and uniformity of structure and chemistry at the centre line of the cast product. Productivity advantages accompany the quality improvements. Experimental results have shown a beneficial effect from electro-magnetic stirring on the microstructure of the steel, for example by increasing the equiaxed zone width. Several types of defects in the strand can be effectively decreased in magnitude with the application of electro-magnetic stirring. Bubbles and porosities are also expected to be considerably affected by the electro-magnetic stirring. It is further reported that the electro-magnetic stirring increases the yield and the productivity of the continuous casting process.
Electro-magnetic stirring can be categorized based on where it is installed in the casting machine. According to the setup position and metallurgical aspects, all electromagnetic stirrers can be classified into three types. These three possible stirrer applications according to the position and the needed effects on the cast steel product are (i) M-EMS, (ii) S-EMS, and (iii) F-EMS. M-EMS is located in the mould, as the name suggest. It is the in-mould stirring (sometimes termed as primary EMS). S-EMS is located below the mould in the secondary cooling region. It is the stirring below the mould where there remains a large percentage of liquid steel (sometimes termed as secondary electro-magnetic stirring or below mould stirring). F-EMS is located at the end of the metallurgical length (just before solidification is complete). It is the stirring just prior to the final solidification point (termed as final electro-magnetic stirring).
Secondary cooling zone – Secondary cooling system consists of several zones. Each zone is meant for a segment of controlled cooling of the solidifying strand as it progresses through the machine. The sprayed medium is either water or a combination of air and water. The heat transfer occurs in this region through all the three ways namely radiation, conduction, and convection. The predominant form of heat transfer in the upper regions of the secondary cooling area is by radiation. As the product passes through the rolls, heat is transferred through the shell as conduction and also through the thickness of the rolls, as a result of the associated contact. This form of heat transfer follows the Fourier law. The third form of heat transfer mechanism occurs by fast moving sprayed water droplets or mist from the spray nozzles, penetrating the steam layer next to the steel surface, which then evaporates. This convective mechanism is as per the Newton’s law of cooling. The heat transfer in the secondary zones serves the functions of (i) enhancing and controlling the rate of solidification (ii) strand temperature regulation through spray water intensity adjustment (iii) machine containment cooling.
Secondary cooling zone in association with a containment section is positioned below the mould, through which the strand, the major portion of which is still in liquid state, passes and is sprayed with water or air mist for further solidification of the strand.
While the strand is continuously withdrawn at the casting speed, solidification of steel continues beneath the mould through the different zones of cooling having a series of water sprays. The secondary cooling system consists of these different zones, each responsible for a segment of controlled cooling of the solidifying strand as it progresses through the continuous casting machine. The sprayed medium is either water or a combination of air and water (mist spray cooling). Mist spray cooling provides the several advantages which include (i) uniform cooling, (ii) less water requirement, and (iii) reduced surface cracking.
Product quality in the continuous casting machine is considerably influenced by temperature variations during strand cooling in secondary cooling zone. Hence secondary cooling zone has a very important function for the maintenance of a correct temperature parameter and is crucial to the quality of the cast steel product.
Since the quality of steel depends on the behaviour of the surface temperature and the solidification of steel front in time, it is to a large extent defined by the intensity of the water sprays. Improper cooling conditions can have detrimental impact on stress distribution in the solidified shell. First of all, overcooling can lead to the formation of cracks. Also, there need to be a smooth transition of the surface temperature as the steel passes through in the secondary cooling zone. In addition, under cooling of the strand during secondary cooling can result in a liquid pool which is too long. These technological requirements demand more efficient and reliable spray cooling and result in constraints which are to be imposed on the secondary cooling process. The spray flow rates are normally adjusted to control the strand surface temperature until the liquid core is solid enough to reach the metallurgical length.
The two mechanisms of overcooling and under cooling also lead to midway cracks and surface cracks respectively. If such quality problems are encountered in a casting operation, a rational basis is needed for changing the settings in the secondary cooling zone, to yield a more satisfactory surface temperature profile.
Strand containment – The containment region is an integral part of the secondary cooling area. A series of retaining rolls contain the strand, extending across opposite strand faces. Edge roll containment can also be needed. The focus here is to provide strand guidance and containment until the solidifying shell is self-supporting. In order to avoid compromises in product quality, careful consideration is needed to be given to minimize stresses associated with the roller arrangement and strand unbending. Hence, roll layout, including spacing and roll diameters are to be carefully selected to minimize between roll bulging and liquid / solid interface strains. Strand support needs maintaining strand shape, as the strand itself is a solidifying shell containing a liquid core which possesses bulging ferro-static forces from head pressure related to machine height. The area of greatest concern is high up in the machine. Here, the bulging force is relatively small, but the shell is thinner and at its weakest. To compensate for this inherent weakness and avoid shell rupturing and resulting liquid steel breakouts, the roll diameter is small with tight spacing. Just below the mould all four faces are typically supported, with only the broad faces supported at regions lower in the machine.
Unbending and straightening – The unbending and straightening section is needed in all the continuous casting machines except in case of straight vertical continuous casting machines. The unbending and straightening forces are as important as strand containment and guidance from the vertical to horizontal plane are important. As unbending occurs, the solid shell outer radius is under tension, while the inner radius is under compression. The resulting strain is dictated by the arc radius along with the mechanical properties of the cast steel grade. If the strain along the outer radius is excessive, cracks can occur. This affects seriously the cast steel quality. These strains are typically minimized by incorporating a multi-point unbending process, in which the radii become progressively larger in order to gradually straighten the product into the horizontal plane.
Severing unit (cutting torches or mechanical shears) – Cutting torches or mechanical shears are needed to cut the solidified strand into pieces of desire length for removal and further processing.
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