Continuous Casting Technology for Ferrous and Non-Ferrous Foundries

FET Faculty (Engineering and Technology) MMU Multimedia University, Malaysia n continuous casting process, metal is achieved and dense bars, semi-finished products with a fine structure Imelted in a furnace and poured into a of crystals and even chemical composition is produced. Continuous tundish. The pour is accomplished through casting allows producing a wide range of different profiles: cylindrical ceramic piping, and the tundish is covered bares, tubes, square bares and tubes, hexagonal profiles, slabs of to prevent oxidation of the metal. From the various thickness and width. This method allows achieving any profile tundish, it is released into a mould. As the form, which is possible to flare through the crystallizer. metal cools and solidifies, usually aided Continuous Casting Technology in Foundries with water cooling systems, which surround An industrial horizontal continuous casting plant is shown below in the mould, finished cast metal protrudes Figure-2. It explains the processing of rod casting and strip casting, from the far end of the mould. Torches can which is self-explanatory. be used to cut the metal off at set lengths FIGURE-1 : INDUSTRIAL CONTINUOUS CASTING PLANT as desired. This method of metal casting is most effective, if it is necessary to manufacture semi-finished products of standardized form in large series. This method also provides increased control over the process through automation. Equal and continuous supply of metal, its crystallization and removal of the product allows obtaining a homogeneous semi- finished metal product during the casting process. With intensified cooling with water, it is possible to increase the speed of crystallization. Choosing the right speed, there can be a focused crystallization in the material, i.e., lengthwise to the cast axis is y g

o Continuous casting transforms that enough heat is extracted to solidify at the casting speed. Liquid metal l

o molten metal into solid on a continuous a shell of sufficient thickness. The shell continues to pour into the mould to

n basis and includes a variety of is withdrawn from the bottom of the replenish the withdrawn metal at an h

c important commercial processes. mould at a "casting speed" that matches equal rate. Upon exiting the mould, the

e These processes are the most efficient the inflow of metal, so that the process strand enters a roller containment T way to solidify large volumes of metal ideally operates at steady state. Below section and secondary cooling into simple shapes for subsequent the mould, water is sprayed to further chamber in which the solidifying strand processing. Most basic metals are extract heat from the strand surface, is sprayed with water, or a combination mass-produced using a continuous and the strand eventually becomes of water and air referred as "air-mist" casting process, including over 500 fully solid when it reaches the to promote solidification. Once the million tons of steel, 20 million tons of ''metallurgical length''. Solidification strand is fully solidified and has passed aluminium, and 1 million tons of copper, begins in the mould, and continues through the straightener, the dummy nickel, and other metals in the world through the different zones of cooling bar is disconnected, removed and each year. Continuous casting is while the strand is continuously stored. distinguished from other solidification withdrawn at the casting speed. FIGURE-2 (B) SCHEMATIC SKETCH OF processes by its steady state nature, Finally, the solidified strand is CONTINUOUS CASTING PROCESS WITH relative to an outside observer in a straightened, cut, and then discharged STRAIGHTENING ROLLS & CUTTING TORCH laboratory frame of reference. The for intermediate storage or hot charged molten metal solidifies against the for finished rolling. Continuous casting mould walls while it is simultaneously process details are shown below in withdrawn from the bottom of the Figure-2 (a) and Figure-2 (b). mould at a rate, which maintains the FIGURE-2 (A) SCHEMATIC SKETCH OF solid-liquid interface at a constant CONTINUOUS CASTING PROCESS 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 generally has a higher capital cost, but lower operating cost. It is the most cost- and energy- efficient method to mass-produce semi finished metal products with consistent quality in a variety of sizes and shapes. Cross- sections can be rectangular, for subsequent rolling into plate or sheet, square or circular for long products, and even “dog-bone” shapes, for rolling into I or H beams. Many different types of continuous casting processes exist. Vertical machines are The shape of the tundish is typically used to cast aluminium and a few other rectangular. Nozzles are located along metals for special applications. Curved its bottom to distribute liquid steel to machines are used for the majority of the mould. The tundish also serves steel casting and require bending and several other key functions such as to or unbending of the solidifying strand. enhance oxide inclusion separation, to Horizontal casting features a shorter provide a continuous flow of liquid building and is used occasionally for To start a cast, the bottom of the steel to the mould during ladle both nonferrous alloys and steel. mould is sealed by a steel dummy bar. exchanges, to maintain a steady metal Finally, thin strip casting is being This bar prevents liquid metal from height above the nozzles to the mould, pioneered for steel and other metals in flowing out of the mould and the thereby keeping steel flow uniform and low-production markets in order to solidifying shell until a fully solidified also to provide more stable stream minimize the amount of rolling strand section is obtained. The liquid patterns to the mould. The main required. poured into the mould is partially' function of the mould is to establish a Co n t i n u o u s C a s t i n g P ro ce s s solidified in the mould, producing a solid shell sufficient in strength to Description strand with a solid outer shell and a support its liquid core upon entry into In the continuous casting process, liquid core. In this primary cooling the secondary spray-cooling zone. molten metal is poured from the ladle area, once the steel shell has a The mould is an open-ended box into the tundish and then through a sufficient thickness, the partially structure, containing a water-cooled submerged entry nozzle into a mould solidified strand will be withdrawn out inner lining fabricated from a high cavity. The mould is water cooled so of the mould along with the dummy bar purity copper alloy. The inner face of

30 June 2013 the copper mould is often plated with are now fully computer-controlled. While the large amount of chromium or nickel to provide a harder Several electromagnetic and thermal automation helps produce castings working surface, and to avoid copper sensors in the ladle shroud, tundish and with no shrinkage and little pickup on the surface of the cast mould sense the metal level or weight, segregation, continuous casting is of strand, which can otherwise facilitate flow rate and temperature of the hot no use if the metal is not clean surface cracks on the product. Mould metal, and the programmable logic beforehand, or becomes 'dirty' during oscillation is necessary to minimize controller, PLC can set the rate of the casting process. One of the main friction and sticking of the solidifying strand withdrawal via speed control of methods through which hot metal may shell, and avoid shell tearing, and liquid the withdrawal rolls. The flow of metal become dirty is by oxidation, which steel breakouts, which can wreak into the moulds can be controlled by occurs rapidly at molten metal havoc on equipment and machine two methods: temperatures (up to 1700 °C); downtime due to clean up and repairs. l By slide gates or stopper rods at inclusions of gas, slag or undissolved Friction between the shell and mould is the top of the mould shrouds alloys may also be present. To prevent reduced with mould lubricants such as l If the metal is open-poured, then oxidation, the metal is isolated from the oils or powdered fluxes. Oscillation is the metal flow into the moulds is atmosphere as much as possible. To achieved either hydraulically or via controlled solely by the internal achieve this, exposed metal surfaces motor-driven cams or levers which diameter of the metering nozzles. are covered by the shrouds, or in the support and reciprocate or oscillate the T h e s e n o z z l e s a r e u s u a l l y case of the ladle, tundish and mould, by mould. interchangeable. synthetic slag. In the tundish, any inclusions in the form of gas bubbles, Continuous Casting Start-up, Overall casting speed can be other slag or oxides, or undissolved Process Control and Problems adjusted by altering the amount of alloys may also be trapped in the slag The mechanism of continuous metal in the tundish, through the ladle layer. casting process is shown below in slide gate. The PLC can also set the Figure-4. Starting a continuous casting mould oscillation rate and the rate of A major problem that may occur in machine involves placing a dummy bar, mould powder feed, as well as the continuous casting is breakout. This is essentially a curved metal beam, up spray water flow. Computer control when the thin shell of the strand through the spray chamber to close off also allows vital casting data to be breaks, allowing the still-molten metal the base of the mould. Metal is poured repeated to other manufacturing inside the strand to spill out and foul the into the mould and withdrawn with the centres particularly the steelmaking machine, requiring a turnaround. Often, dummy bar once it solidifies. It is furnaces, allowing their work rates to breakout is due to too high a extremely important that the metal be adjusted to avoid 'overflow' or withdrawal rate, as the shell has not supply afterwards be guaranteed to 'underrun' of product. had the time to solidify to the required avoid unnecessary shutdowns and thickness, or the metal is too hot, which restarts, known as 'turnarounds'. FIGURE-3 : MECHANISM OF means that final solidification takes Each time the caster stops and CONTINUOUS CASTING PROCESS place well below the straightening rolls restarts a new tundish is required, as and the strand breaks due to stresses any uncast metal in the tundish cannot applied during straightening. A be drained and instead freezes into a breakout can also occur if solidifying 'skull'. Avoiding turnarounds requires steel sticks to the mould surface, the melt shop, including ladle furnaces causing a tear in the shell of the strand. to keep tight control on the If the incoming metal is overheated, it temperature of the metal, which can is preferable to stop the caster than to vary dramatically with alloying risk a breakout. Additionally, lead additions, slag cover and deslagging, contamination of the metal caused by and the preheating of the ladle before it counterweights or lead-acid batteries a c c e p t s m e t a l , a m o n g o t h e r in the initial steel charge can form a parameters. thin film between the mould wall and the steel, inhibiting heat removal and However, the castrate may be shell growth and increasing the risk of lowered by reducing the amount of breakouts. metal in the tundish, although this can increase wear on the tundish, or if the Another problem that may occur is a caster has multiple strands, one or carbon boil, i.e., oxygen dissolved in more strands may be shut down to the steel reacts with also-present accommodate upstream delays. carbon to generate bubbles of carbon Turnarounds may be scheduled into a monoxide. As the term boil suggests, production sequence if the tundish this reaction is extremely fast and temperature becomes too high after a violent, generating large amounts of certain number of heats. hot gas, and is especially dangerous if it occurs in the confined spaces of a Many continuous casting operations casting machine. Oxygen can be y g

o removed through the addition of silicon upon the shell strength. Minimisation of transfer not only limits maximum l

o or aluminium to the steel, which reacts the friction and continuous release of productivity but also profoundly

n to form silicon oxide (silica) or the shell from the mould have been influences cast quality, particularly h

c aluminium oxide (alumina). However, achieved through the introduction of with respect to the formation of surface

e too much alumina in the steel will clog mould oscillation aided by lubrication. and internal cracks. In part, this is T the casting nozzles and cause the steel Heat Transfer in Continuous Casting because metals expand and contract to 'choke off'. By its nature, continuous casting is during periods of heating or cooling. Computational fluid dynamics and primarily a heat-extraction process. That is, sudden changes in the other fluid flow techniques are being The conversion of molten metal into a temperature gradient through the solid used extensively in the design of new solid semi-finished shape involves the shell, resulting from abrupt changes in continuous casting operations, removal of the following forms of heat. surface heat extraction, causes differential thermal expansion and the especially in the tundish, to ensure that l Superheat from the liquid entering generation of tensile strains. inclusions and turbulence are removed the mould from the tundish. from the hot metal, yet ensure that all Depending on the magnitude of the l The latent heat of fusion at the the metal reaches the mould before it strain relative to the strain-to-fracture solidification front as liquid is cools too much. Slight adjustments to of the metal and the proximity' of the transformed solid, and finally. the flow conditions within the tundish strain to the solidification front, cracks or the mould can mean the difference l The sensible heat, i.e., cooling may form in the solid shell. The rate of between high and low rejection rates of below the solidus temperature from the heat extraction also influences the the product. solid shell. ability of the shell to withstand the Solidification Control in Continuous These heats are extracted by a bulging force due to the ferrostatic Casting of Steel combination of the following heat- pressure owing to the effect of transfer mechanisms: temperature on the mechanical Solidification in continuous casting properties of the metal. Therefore, technology is initiated in a water- l Convection in the liquid pool. heat transfer analysis of the continuous cooled, open-ended copper mould. l H e a t c o n d u c t i o n d o w n casting process should not be The steel shell, which forms in the temperature gradients in the solid shell overlooked in the design and operation mould, contains a core of liquid steel, from the solidification front to the of a continuous casting machine. which gradually solidifies as the strand colder outside surface of the cast. Comparison of continuous casting moves through the caster guided by a l External heat transfer by process and ingot casting primary large number of roll pairs. The radiation, conduction and convection to rolling process is shown in Table-1. solidification process initiated at surroundings. meniscus level in the mould is Because heat transfer is the major Advantages of Continuous Casting completed in secondary cooling zones phenomenon occurring in continuous Technology using a combination of water spray and casting, it is also the limiting factor in l Reduced cost. radiation cooling. Solidification the operation of a casting machine. The l Improved quality. speciality of continuous casting distance from the meniscus to the cut- l Less variability in chemical technology arises from the dynamic off stand should be greater than the composition both along the thickness nature of the casting process. In metallurgical length, which is and along the length and surface has particular this relates to handling of dependent on the rate of heat fewer defects. very high heat flux in the mould, conduction through the solid shell and l Increased yield, since it is not nurturing of the initial thin and fragile of heat extraction from the outside necessary to crop the ends of solid shell for avoidance of breakout surface, in order to avoid cutting into a continuously cast slabs. during descent of the strand down the liquid core. Thus, the casting speed l Reduced energy costs. mould, designing of casting parameters must be limited to allow sufficient time in tune with the solidification dynamics for the heat of solidification to be l The slabs are sent directly to hot of the steel grade for minimisation or extracted from the strand. Heat rolling and do not require pits for elimination of surface and internal defects in the cast product. Safe caster TABLE-1 : CONTINUOUS CASTING VS INGOT CASTING operation, i.e., without metal breakout Continuous Casting Ingot Casting – and achievement of acceptable product Process Primary Rolling Process quality require understanding of both Productivity (Molten Steel to Slab) 30 to 60 min 1 to 2 Days process engineering and metallurgy of solidification. Early solidification in Yield (Molten Steel to Slab) 96 to 99% 80 to 90% continuous casting occurs in the form Energy Saving 1/2 to 1/4 Base of partial freezing of the meniscus Labor Saving 6 to 10 Persons / Shift 20 to 30 Persons / Shift curvature originating from the mould liquid contact point. To minimise shell Plant Floor Space Saving 1/3 or Less Base sticking and tearing, friction between Steel quality and the homogeneity of quality are better the strand surface and mould wall must Miscellaneous than achievable by ingot casting primary rolling process, be kept below a critical level depending and heavy work load is greatly alleviated.

32 June 2013 reheating and also the thickness of this there is less possibility for arising products by changing the cast continuously cast slabs is half the of tension in the product and the parameters (the pulling speed of the thickness of ingot castings and thus finished parts made from such products product, the temperature of the require lower energy for hot rolling. will be more durable; crystallizer's water); l Less pollution. l Continuous casting reduces metal l Continuous casting allows to l The dimensions produced in cuttings and loss of metal compared to manufacture metal slabs or bars in continuous castings are more casting in moulds; large amounts by short time. amenable for hot rolling. l Continuous casting is more Conclusion l In continiuos casting process, the economic regarding the energetic Continuous casting technology is a ladle has to move short distance i.e. consumption; metalworking process in which metal is only over the tundish and not over l There are less workers needed at cast continuously, rather than being every mould. the production process compared to cast in discrete molds. This process is l Continuous casting is perfect for casting in moulds; extremely efficient and cost effective, manufacturing semi-finished products l All operations can be easily making it popular for the production of (bars, slabs) of long sizes; automated and supervised. a variety of semi finished metal shapes. l The microstructure of long semi- l It is easy to modify the quality and Once cast, the metal can be further finished products is even and due to properties of the semi-finished worked as needed.