ASM Metals Handbook - Desk Edition (ASM_ 1998) Episode 3 pps

microcleanliness. If the furnace is also equipped to vacuum degas, it can additionally desulfurize the steel. Gas stirring or induction stirring is used in addition to arc reheating as a prominent process design variation. The Finkl-Mohr VAD degassing system uses gas stirring whereas the ASEA-SKF ladle refining furnace uses induction stirring. Gas stirring is designed for desulfurization and other steelmaking refinements including degassing. The process is similar to a ladle vacuum degassing method described in the subsection "Secondary Steelmaking Equipment," except that the steel can be heated before or after the degassing operation using an electric arc provided by electrodes inserted through the vacuum tank cover (Fig. 14). An inert gas is used to stir molten steel in the ladle degassing process. Fig. 14 Schematic arrangement of equipment used in the gas-stirring, arc-reheating process When induction stirring is performed (ASEA-SKF process), the system comprises a ladle furnace, a mobile induction coil, a vacuum cover with exhaust line, a steam ejector system, and a cover fitted with three carbon electrodes. Sections of the vessel shell are produced from nonmagnetic austenitic stainless steel. The same ladle can be used to tap the heat from the converter and then serve as a heating furnace, vacuum vessel, and teeming ladle. The steel is tapped without superheat and without any deoxidant addition. Slag-free tapping is performed. The ladle is placed in the mobile induction coil and covered with the top containing the three electrodes. The arcs are struck to begin the reheating and refining period. Fluxes are added to prepare a basic slag, and alloy additions are made to meet the compositional specifications. Sulfur is reduced to below 0.005%. Heat loss is restored and the vessel is ready for degassing. The cover is replaced with the vacuum furnace cover, and the vessel is evacuated. Induction coils are energized to stir the steel. Aluminum and silicon are added toward the end of degassing. Following the degassing, the same ladle is moved to the teeming platform for ingot casting, or the steel is poured into a tundish for continuous casting (see the section "Casting" ). Vacuum-Arc Remelting Consumable-electrode melting under vacuum, or VAR, is the refining process used for special-quality steels and stainless steels that are first made by conventional steelmaking methods and subsequently cast or forged into electrodes for vacuum drip melting into a water-cooled copper mold under very low pressures of 0.1 torr. Because of the high arc temperature and the small pool of liquid metal, sound ingots with dense crystal structure, low hydrogen and oxygen contents, and minimal chemical and nonmetallic segregation are produced. Direct current is employed for melting. The diameter of the electrode and its relationship to the crucible is critical and must be matched for the melting rate. Melting rates as high as 1150 kg/h (2500 lb/h) are used to produce ingots as large as 1.5 m (5 ft) in diameter. Nonvacuum Refining Secondary steelmaking processes used for special and clean steel production and carried out at atmospheric pressure without any supplemental reheating include argon bubbling processes such as capped argon bubbling (CAB), composition adjustment by sealed argon bubbling (CAS), argon-oxygen decarburization (AOD), ESR, and ladle injection methods. (Ladle injection methods are described in the sub-section "Secondary Steelmaking Equipment." ) Argon Bubbling Processes. Argon bubbling argon stirring, trimming, and rinsing is used for quick and uniform mixing of alloys, temperature homogenization, adjustment of chemical composition, and partial removal of nonmetallic inclusions. These functions are accomplished by either blowing argon through a refractory-protected lance lowered to within 300 mm (12 in.) of the ladle bottom or by blowing argon through porous refractory plugs inserted in the bottom or side wall of the ladle. Argon bubbling often supplements other secondary steelmaking operations by promoting bulk movement of steel in the ladle for chemical and thermal homogeneity, enhancing the flotation of inclusions, and promoting intimate metal-slag mixing for refining operations, such as desulfurization and deoxidation. Argon stirring is the most vigorous of the bubbling treatments, injecting the highest flow rates of up to 0.3 m 3 /min (10 scfm) through liquid steel. Stirring, which usually follows tapping, is used to mix the slag and metal and for temperature and chemical homogenization. The high flow rate breaks through the top synthetic slag layer and creates an opening for alloy and deoxidant additions. Sometimes radiant heat losses are promoted during stirring to prepare the heat for continuous casting. Carry over of slag from the converter should be minimized. Argon rinsing follows the stirring to help float the inclusions into the slag, and the gas flow rate is below 0.15 m 3 /min (5 scfm). During argon rinsing, the gentle flow rate of argon prevents the formation of new heat-radiating surfaces. If the steel composition requires adjustment through ferroalloy additions, it is carried out during argon trimming which occurs between the stirring and rinsing steps. Just enough gas flow (0.15 to 0.3 m 3 /min, or 5 to 10 scfm) during the trimming period allows the ferroalloys to mix in the steel and not become lost in the slag layer. The specified rates in this description are somewhat dependent on the ladle size. The CAB and CAS methods for argon bubbling were developed to make controlled additions to the ladle as well as to improve the steel refining capability. The CAB process uses a conventional ladle with a cover and requires a synthetic slag over the steel surface after tapping (Fig. 15a) to act as a sponge for the absorption of nonmetallic inclusions. The introduction of argon into the covered ladle through a porous bottom plug stirs the metal vigorously and creates a slag- metal mix, unlike other argon bubbling methods, which require an intact slag layer to protect the melt from oxidation. The cap on the ladle prevents any air from affecting the metal. The slag-metal emulsion is useful in enhancing microcleanliness, chemical homogenization, desulfurization, and deoxidation. The CAS process (Fig. 15b) uses a refractory-lined snorkel, which is lowered inside the melt during argon stirring so the steel inside the snorkel is slag free. This allows the addition of ferroalloys and deoxidizers without any slag interference and, therefore, is an effective secondary steelmaking process for achieving compositional control. Fig. 15 Schematic arrangement of equipment used in argon- bubbling processes. (a) Capped argon bubbling (CAB) process. (b) Composition adjustment by sealed argon (CAS) bubbling process The AOD process was designed for economical production of chromium-bearing stainless steels. A premelt is prepared in an electric-arc furnace by charging high-carbon ferrochrome, ferrosilicon, stainless steel scrap, burned lime, and fluorspar and melting the charge to the desired temperature. The heat is then tapped, deslagged, weighed, and transferred into an AOD vessel, which consists of a refractory-lined steel shell mounted on a tiltable trunnion ring (Fig. 16). As shown in Fig. 16, process gases (oxygen, argon, and nitrogen) are injected through submerged, side-mounted tuyeres. The primary aspect of the AOD process is the shift in the decarburization thermodynamics that is afforded by blowing with mixtures of oxygen and inert gas as opposed to pure oxygen. Fig. 16 Schematic of argon oxygen decarburization vessel The heat is decarburized in the AOD vessel to 0.03% C in stages during which the inert gas to oxygen ratio of the blown gas increases from 1-to-3 to 3-to-1. During the blowing, fluxes are added to the furnace and a slag is prepared. Following the decarburization blow, ferrosilicon is added and the heat is argon stirred for a short period. The furnace is then turned down, a chemistry sample is taken, and the heat is deslagged. Additional alloying elements are added if adjustments are necessary, and the heat is tapped into a ladle and poured into ingot molds or a continuous casting machine. With the AOD process, steels with low hydrogen (<2 ppm) and nitrogen (<0.005%) can be produced with complete recovery of chromium. The ESR process, like VAR, is a secondary refining process for electrode ingots of essentially the same composition as the finished product, except that ESR is carried out at normal atmospheric pressure and has a greater melting rate than VAR. Rotor forgings, rolls, molds and dies, nuclear containment vessels, and special casting shapes are produced by ESR. Alloy steels, stainless steels, and nickel-base superalloys are also commonly produced using ESR facilities. Electroslag remelting units consist of an open-bottom, water-cooled copper mold that contains the molten slag and metal, a high- current, low-voltage ac or dc power source, and an electrode feed mechanism. The mold rests on the starting plate at the beginning of melting and gradually moves upward as melting progresses (Fig. 17). Castings can be produced generally in any geometrical shape. The arc is struck between the base plate and the electrode to melt the slag, which is electrically conductive. As the electrode tip melts in the form of droplets and passes through the slag layer, some refining occurs. As melting proceeds, the molten pool of metal gradually solidifies. The rate is adjusted such that a molten pool depth equal to one-half the electrode diameter is maintained. The slag composition can be adjusted to serve as a desulfurizer or dephosphorizer as well as a reservoir for floating inclusions. Sulfur can be lowered to below 0.002%. The slag is comprised of fluorspar, lime, and alumina. The ESR process is ineffective in lowering hydrogen, however, as the process is essentially atmospheric. The ESR process is capable of using multiple electrodes, which can be melted into a single mold. The product surface quality is excellent and requires no conditioning. Fig. 17 Schematic of the electroslag remelting process Reference cited in this section 35. W.T. Lankford, Jr., et al., Ed., The Making, Shaping and Treating of Steel, 10th ed., US Steel Publication, 1984, p 664-669 Casting After making and refining by a selected combination of process steps described previously, the steel is then ready to be cast. The ladle containing the refined steel is equipped with either a stopper-rod arrangement or a slide-gate system to control the flow of steel during teeming into a series of molds or a continuous casting tundish. Pit-side practice is one of the key activities in steelmaking, which includes the final process steps before the steel is solidified by the ingot casting method. Therefore, the steel quality is significantly influenced by the pit-side practice adopted for a given steel grade. The ingots are stripped from the molds after the steel is almost completely solidified. Solid steel ingots are soaked in a heated pit where the temperature is homogenized for primary rolling into semifinished products of blooms, slabs, or billets. An attempt is made to reduce the "track-time" between stripping and inserting into soaking pits to conserve energy. Alternatively, a refined steel ladle can be directly teemed into a continuous casting tundish, which supplies molten steel for direct casting into semifinished or near-net shape steel cross sections. Ingot Casting An ideal ingot is characterized by a homogeneous chemical and physical structure with fine equiaxed crystals and with no chemical segregation and nonmetallic inclusions. However, depending on the nature of casting and solidification processes, ingots develop pipe, blowholes, chemical and nonmetallic segregation, internal fissures, and columnar crystal structure to varying extents. In addition, surface scabs and panel cracks also commonly develop on ingot surfaces. Ingot molds are usually tapered rectangular boxes of refractory-lined cast iron and are used in both orientations of "wide end up" or "wide end down." In the wide-end-down arrangement, molds may have an open top or a bottle top, whereas wide- end-up molds are generally equipped with open, closed, or plug bottoms. Casting of special high-quality steels presently employs wide-end-up configuration. The inner walls of the mold can be plain sided, cambered, corrugated, or fluted. Corrugating or fluting is done to minimize surface cracking of ingots during solidification by promoting a faster cooling rate and forming a thicker initial ingot skin. The liquid steel is fed into the mold either by top pouring or bottom pouring through refractory-lined feeders. The size and shape of an ingot is closely linked with the yield at the slabbing mill. As the mold is being filled with steel, an ingot shell or skin forms next to the mold walls and bottom. This skin contracts as it cools to form an air gap between the mold wall and the solidified shell. Formation of an air gap reduces the rate of heat extraction from the ingot. As steel solidifies, the thermal gradient also becomes less steep and the rate of ingot skin formation slows down. The solubility of gases in molten steel decreases with decreasing temperature causing liberation of gases. Oxygen predominantly escapes as carbon monoxide after reacting with carbon. The amount of dissolved oxygen is decreased by the addition of deoxidizing agents. It is also dependent on the carbon level and chemical composition of the steel. The degree of deoxidation achieved during ingot solidification establishes four classes of steel: killed, semikilled, capped, and rimmed. The rate of heat extraction from an ingot is affected by thickness, shape, and temperature of the mold, the amount of superheat in the liquid steel, the ingot cross section, and the type and chemical composition of the steel. Ingot size is selected to meet the product requirement and the capabilities of the hot-working facility. Ingots range from a few hundred kilograms in weight to as high as 300 metric tons for large forgings. An ideal cooling profile for a killed-steel ingot is shown in Fig. 18 as a function of time. Fig. 18 Ideal solidification pattern of a hot-topped, wide-end-up ingot of fully killed steel Deoxidation Practices. Because steel solidifies over a temperature range and as the carbon-oxygen chemical equilibrium is constantly changing with temperature, the carbon monoxide gas evolved from still liquid portions as a result of the new equilibrium condition may become trapped at solid-liquid interfaces to produce blowholes. The type of ingot structure is controlled by the degassing allowed during solidification. Figure 19 shows a series of ingot structures for a bottle-top mold casting, ranging from fully killed or dead-killed ingot (No. 1) to a violently rimmed ingot (No. 8) (Ref 36). The fully killed ingot, where no gas evolution is allowed due to full deoxidation through deoxidizer additions, is characterized by the intermittently bridged shrinkage cavity known as pipes. Fully killed steels are commonly cast in wide-end-up molds with hot tops to confine the pipe cavity near the hot top portion. Exothermic compounds are also used in killed ingot casting to allow flotation of nonmetallic inclusions and to keep the steel molten at the top. In a semikilled steel (No. 2), carbon monoxide is allowed to evolve slightly where the resulting blowholes compensate for the solidification shrinkage. The ferrostatic head helps keep the bottom half of the ingot free of blowholes, and the top begins to bulge due to the pressure exerted by the trapped gases. In ingot No. 3, more carbon monoxide is allowed to evolve resulting in greater volume of blowholes than required to compensate for the shrinkage. Some of the blowholes formed close to the side surface in the top half of the ingot are detrimental to the surface quality and lead to surface defects, known as seams, during subsequent hot working. The gas pressure punctures the initially frozen top surface and forces liquid steel up through the rupture causing bleeding. Excessive bleeding results in a spongy surface on products rolled from such ingots. Fig. 19 Eight typical conditions of commercial steel ingots, cast in identical bottle- top molds, in relation to the degree of suppression of gas evolution. The dotted line indicates the height to whic h the steel originally was poured in each ingot mold. Depending on the carbon and, more importantly, the oxygen content of the steel, the ingot structures range from that of a fully killed ingot (No. 1) to that of a violently rimmed ingot (No. 8). Ingot No. 5 represents a typical capped ingot where numerous honeycomb blowholes extended from top to bottom as a result of strong gas evolution. This evolution causes the steel surface to rise after pouring, and a boiling action ensues, known as rimming action. This action is stopped by using a metal cap on the top of the mold. In this case, a thick, solid skin forms as blowholes are swept upward by the evolving gas. As the evolution slows down, honeycomb blowholes appear in the middle of the ingot. These blowholes do not pose any surface defect problems during rolling. Ingot No. 7 is a typical rimmed steel ingot structure where gas evolution is so strong that blowholes are confined to the lower quarter of the ingot only. The apparent increase in volume due to blowholes offsets the shrinkage that occur during solidification, thus, causing a rather flat-top ingot surface. A description of the production of different types of ingots, including the formation of blowholes and pipes that occur in ingots as a result of the chosen ingot casting practice, appears in greater details in Ref 37. Major Casting Defects. Several other casting defects occur in steel ingots, such as segregation, inclusions, columnar structure growth, fissures, internal and surface cracks, and scabs. The type and size of ingot and the chemical composition of the steel primarily influence segregation in steel castings. The first metal to solidify very close to the mold wall, namely the chilled zone, has very similar chemical composition to that of the poured steel. However, as solidification proceeds at a decreasing rate, dendrite crystals of purer metal, which are low in carbon, manganese, silicon, sulfur, and phosphorus and other elements, solidify first. The dendrite crystals reject these elements into the remaining liquid. The last material to solidify contains the largest amount of these rejected elements. Segregation is commonly expressed as the departure from an average chemical composition in a bulk material. A positive or negative segregation refers to an increase or a decrease of an element from the average composition, respectively. Sulfur has the greatest tendency to segregate followed by phosphorus, cabon, silicon, and manganese. Larger ingots, which take a longer time to solidify, show greater segregation. When steel is stirred during solidification by convection currents, or turbulence due to gas evolution, the tendency to segregate is enhanced. Thus, a killed steel shows minimum segregation, whereas a rimmed steel shows a sharp boundary between the negatively segregated rimmed zone and a positively segregated core zone. In killed steels, however, "V" segregation occurs along the central axis of the ingot giving rise to axial porosity. Inverted "V" or "A" segregation occurs as a result of ingot disturbance during solidification and gives rise to a defect known as ingot pattern (Ref 38). The chill zone, or the first formed layer of steel adjacent to the mold wall, has a small and randomly oriented crystal structure, followed by a large dendritic crystal growth characterized by a branching structure. Growth of the individual dendrites occurs principally along the longitudinal axes perpendicular to the ingot surface and can extend all the way to the center of the ingot. An ingot predominantly possessing these large elongated dendritic crystals is referred to as having a columnar structure. Such ingots tend to crack excessively if heavy reductions are taken during initial rolling passes. Usually, columnar structure gives way to the formation of large, equiaxed, randomly oriented dendritic crystal structure toward the center of the ingot. The relative proportion of columnar and equiaxed dendritic crystal structure depends on the steel composition, mold temperature, pouring temperature, and gas content of the steel. Movement of liquid steel during solidification is sometimes practiced by various mechanisms to decrease or eliminate the formation of columnar dendritic zone (Ref 39). This is achieved by the removal of all superheat in the liquid core, that is, to reduce the liquid core temperature to the steel liquidus temperature and by generation of nuclei fragments in the liquid core. Nuclei fragmentation is achieved by either remelting the columnar dendrite tip or by mechanical breaking. Long columnar crystals, especially in higher-alloy grades that resist plastic flow at hot-rolling temperatures, are undesirable because of poor cohesive strength (Ref 40). Large internal bursts or fissures can be produced by the tensile stresses generated in the interior of the ingot by heating, cooling, or rolling process. If sufficient hot working is performed, these fissures can be completely welded if they do not extend to the surface. Longitudinal as well as transverse cracks in the ingot wall can be seen on the surface of a cold ingot or during primary rolling. These ingot cracks are caused by excessively high pouring temperature. Weak interdendritic zones are formed that extend from the surface to the center of the ingot. A larger number of shorter dendrites develop if the pouring temperature is low. Transverse ingot cracking is also caused by discontinuities in the ingot wall arising from the surging molten metal in the mold. Improved mold design and use of mold coatings help the formation of folds due to a liquid surge. The occurrence of transverse cracking is lowered as carbon content increases in the steel. A hanger crack can be produced as a transverse crack when fins are formed over the edge of the mold. Corner design of the mold or the use of fluted molds give rise to longitudinal cracks. In general, these types of casting defects can be reduced by a proper mold and hot-top design. Molds cast in cement for ingot casting have also shown improved crack-free ingot surfaces. Oxidized materials and sulfides, usually in combination, give rise to nonmetallic inclusions in the steel. These are deoxidation products in most cases, that were not removed during secondary steelmaking or during pouring. In some cases, these inclusions are added into the steel by the erosion of refractory linings used in ladles, furnaces, or molds. In top-poured ingots, the pouring stream strikes the mold bottom and splashes against the lower mold walls. These splashes adhere and solidify forming a layer on the lower portion of the mold wall. As the liquid level rises, splashing diminishes. However, if the rate of rise for the molten pool is slow, the cooled splashed material oxidizes and attaches to the ingot surface as imperfectly bonded scabs. The thick scabs fold in by the rising steel level and, during rolling, give rise to seam or sliver type defects. In flat products, subsurface cracks occur parallel to these folds and produce surface laminations. A faster pouring rate can usually eliminate this kind of defect. Bottom-poured steels also do not have these cracks as the steel entering through the bottom in the mold does not splash and rises uniformly. Ingots can be scalped to remove major surface defects. The minor defects, however, are removed during the roughing (initial) passes as the scale is broken and removed during primary rolling. Continuous Casting The process of continuous production of semifinished shapes of blooms, billets, slabs, and rounds directly by solidification of refined liquid steel is termed continuous casting. The molten steel is fed into a steel reservoir called a tundish and transferred via nozzles into a continuous casting mold. Semifinished product yields as high as 95% can be realized in continuous casting as opposed to approximately 80% yield in the ingot casting/primary rolling route. Presently, 65 to 70% of finished steel is continuously cast worldwide. The combination of an electric-arc furnace and a small continuous billet, bloom, or slab caster, has lead to the formation of minimills characterized by their economic efficiency and simplicity. The first commercial billet and slab casters were installed in the early 1950s. Steel quality is also improved by continuous casting because better control of steel cleanliness can be generally administered in the tundish and the mold and favorable solidification structures obtained through controlled cooling. High-quality clean steels are also produced through ingot casting, particularly the high-carbon, low-oxygen grades. For special grades, like bearing steels, ingot casting is the preferred route because primary rolling allows a larger reduction ratio through hot working and, therefore, a more sound internal structure. A 30 to 1 reduction ratio is required as a minimum for certain bearing grades, which is not achievable from continuously cast billets, or even blooms, if large bearings are to be manufactured. The yield improvement in continuous casting over ingot casting is primarily due to the elimination of scrap generation in three areas: primary rolling mill, steel pouring practice, and soaking-pit ingot heating. In addition, short ingots, ingot butts, and general pit scrap lower the ingot casting yield. Cropping of top and bottom parts in an ingot due to piping or high inclusion level is always associated with ingot casting. Conversely, it can be easily understood that the longer a continuous caster operates without an interruption (number of heats continuously processed), the higher is the yield for any given casting size and number of strands on a caster. Quality improvement ensues from the fact that there is less variability in chemical composition and better solidification characteristics. Segregation is minimized, both vertically along the length of the billet or bloom and across the cross section. Inclusion levels, however, could be higher in continuously cast steels, particularly in transition zones (material cast during change of heats), and as a result are detrimental to bearing-grade steels, as discussed earlier. Further down the line, surface dressing requirements prior to finish rolling are also reduced as surface defects (seams, scabs, etc.) on a continuously cast product are less than a primary rolled product from ingots. This also improves the yield. In general, fewer internal and surface defects are present in continuously cast material. Elimination of soaking and primary rolling steps considerably improves the energy efficiency of continuously cast product. Higher yield in continuous casting lowers the energy consumption per ton of steel processed. With the advent of hot charging of semifinished products from a caster into reheating furnaces prior to finish rolling, energy efficiency of continuous casting has been greatly enhanced. Soaking pits for ingot reheating burn fuel and are a source of pollution. Lower capital and operating costs are required in continuous casting by eliminating ingot processing steps and by yielding higher throughputs (Ref 41). Process Description. Figure 20(a) shows the main components of a continuous caster. A casting machine essentially consists of a liquid-metal reservoir and a delivery system, known as a tundish, a water-cooled mold, a secondary cooling zone with a containment section, bending and straightening rolls, shearing equipment, and a cooling bed or run out table. A caster can have several strands (number of liquid streams tapped from the same tundish), each equipped with a mold, secondary cooling and containment arrangement, shearing station, etc. The number of strands used is a function of the heat size and the shape being cast. Fig. 20 Continuous casting. (a) Major components of a continuous ca sting machine. (b) Liquid metal flow from the ladle into the tundish and from the tundish into the mold The casting process begins with the bottom end of the mold plugged with a dummy bar connected to an external mechanical withdrawal system. The tundish is filled to a certain height at a controlled rate by refined molten steel poured from ladles. The liquid steel flows from the tundish through nozzles and into the mold (Fig. 20b). When the steel level reaches a certain height in the mold, the dummy bar is withdrawn at a predetermined casting speed. Casting speed is dependent on the machine characteristics, such as cast section, cooling efficiency, metal feed rate from the tundish, and the desired cast structure. When the dummy bar head, which is now attached to the solidified shape being cast, reaches a certain position in the withdrawal station, it is mechanically disconnected and the dummy bar removed. The solidified casting continues through the withdrawal system to the shearing station. Solidification begins in the water-cooled mold just below the liquid steel meniscus where a shell is formed in contact with the mold wall. The distance between the meniscus level and the point of complete solidification is known as the metallurgical length of the caster. The mold is vertically oscillated to prevent sticking of the solidified shell to the mold wall. In addition, molds are tapered to ensure that the solidified shell is in contact with the mold wall for better cooling efficiency. Friction between the mold wall and the solid shell is minimized by using mold compounds or lubricants, such as oil or fluxes that form a fluid slag. Casting conditions are established to ensure that the steel shell is thick and strong enough to withstand the ferrostatic pressure of molten steel in the mold after it leaves the mold. The material is fully solid before it reaches the cutter and in many cases, it is solid before it arrives at the straighteners. Further heat removal occurs in the secondary cooling zone for complete solidification. Water and/or mist spray cooling is employed in this zone to maintain optimum cooling rates and strand surface temperatures. Support rolls guide the strand as well as prevent sectional bulging due to internal ferrostatic pressure from the molten pool at the strand core. Cooling sections are designed to minimize internal and external flaws or defects. After the secondary cooling, bending and straightening are performed before shearing into the desired length for further processing, either in hot or cold condition. Some of the casters can be used to cast more than one shape by changing the mold (Ref 42). Capital cost of installation has been lowered over the years while improving the quality of the cast product by progressively reducing the height of the machine. Older installations are vertical machines with a straight mold and cut off in the vertical position whereas newer installations use a bow-type machine with curved mold and progressive straightening. New slab casters are usually bow type because slabs are not self supporting in the secondary cooling zone, whereas billets or blooms are self supporting and, therefore, can be cast on a vertical machine. Generally, the shape of the cast material, productivity and quality of the product, and the cost determine the type of machine chosen. Design Features in Continuous Casting for Quality Temperature control is more critical in continuous casting than in ingot pouring. Enough superheat must be maintained to allow the molten steel ladle travel from tapping to teeming stations and to prevent freezing at the tundish nozzles. At the same time too much superheat can cause insufficient solidification in the mold and thus a breakout after leaving the mold. Low superheat casts provide better uniform cast structures with a wider equiaxed crystal zone than high superheat casts. Homogenization of temperature is, therefore, practiced either by argon bubbling through a porous bottom plug or by lancing from the top in the ladle before the steel is teemed into a tundish. Continuously cast steels must be fully killed (deoxidized) to prevent blowholes or pinholes from forming close to the surface of the cast product, which result in seams upon subsequent rolling. Full deoxidation is achieved primarily by silicon deoxidation for coarse-grain steels and by aluminum deoxidation for fine-grain steels. Aluminum-killed steels, however, can cause problems at the tundish nozzle by clogging them with alumina deposits. High-quality products commonly use a ladle refining practice prior to casting and take special measures for preventing nozzle blockage. Liquid steel is fed continuously or semicontinuously from the ladle to the tundish and is distributed to individual molds through nozzles in a continuous stream (Fig. 20b). Stopper rod or hydraulically or electrically controlled slide-gate systems are used to transfer steel from the ladle to the tundish. Role of the Tundish. Tundishes have nozzles located at the bottom, which serve primarily as a metal distributor to the mold. Metal flow patterns are very critical to the product quality. Flow-control devices, such as refractory dams and weirs, are attached to the tundish to distribute the metal flow, to minimize turbulence, and to eliminate dead flow zones (Ref 43). These devices enhance the stability of the metal streams entering the casting mold. Significant cold modeling work has preceded the design of optimal configuration of these flow-control devices. Metal is poured as far away as possible from the nozzle location directly on a wear resistant pad. A constant metal height above the tundish nozzle is required to discharge the metal at a constant rate and, in turn, to maintain constant casting speeds. Tundishes also perform the function of a metal reservoir which allows unabated casting during ladle change overs for sequence casting of heats. It is imperative that the ladle change be done in the shortest possible time. The metallurgical role of a tundish is to facilitate separation of inclusions and slag from entering the mold. The metal residence time in the tundish is a key parameter in meeting this condition. Tundishes are preheated prior to metal pouring and are often covered to minimize radiation heat losses. Tundish nozzles are either a metering (or open) nozzle or a stopper-rod nozzle. Metering nozzles essentially control the metal discharge rate by the bore of the nozzle and the ferrostatic pressure (metal height in the tundish) above the nozzle. They are commonly used for silicon-killed billet or bloom castings. Stopper-rod nozzles are used for slab casting of aluminum-killed steel, and the flow is monitored by raising or lowering the rod above the nozzle opening. Alumina buildup and clogging is compensated by raising the rod if other means of preventing buildup are not in place. Inclusion levels in the metal rise, particularly during ladle change overs. The time available for floating inclusions is much shorter in continuous casting than in ingot casting. However, nozzle clogging is prevented by bubbling argon through the stopper head and nozzle units in modern installations. Slide- gate systems additionally provide the capability for changing nozzles during casting as well as changing nozzle size. Shrouding. Stringent surface and cleanliness requirements placed on special-quality steels have necessitated the use of closed stream or shrouded castings. Open-stream castings pick up oxygen- and nitrogen-forming inclusions, which have deleterious effects on steel properties, as described earlier. Ladle-to-tundish and tundish-to-mold shrouding are commonly employed to protect the steel from air, particularly the aluminum-killed grades, which have a potential of forming alumina inclusions. Inert-gas shrouding and refractory-tube shrouding are the general types of methods used. Refractory-tube shrouds are made of fused silica or alumina graphite. Figure 21 shows an example of a ladle-to-tundish and tundish-to- mold refractory-tube shrouding system. Fig. 21 Schematic of a refractory-tube shrouding system for minimizing oxidation during pouring Mold Characteristics. The mold serves the function of partly solidifying the steel to such an extent that the shell thickness and shape, temperature distribution, and surface and internal structures are appropriate after exiting. Molds are open-ended boxes that contain an inner lining made of a copper alloy and are externally supported by a steel structure. Cooling water flows between the liner and the outer steel structure to extract heat from the solidifying steel in contact with the copper liner. Molds are either tubular or plate type. One-piece tubular thin copper linings are used for smaller billet and bloom casters. Plate molds consist of a four-piece copper lining attached to steel plates. In some designs, opposite plates can be adjusted to cast different rectangular slab shapes and, therefore, are more adaptable. Plate molds also allow the change of taper to accommodate different shrinkage characteristics of different steel grades. Silver-alloyed copper is used for high-temperature strength. The inside liner surface is often nickel or chrome plated to provide a harder working surface and to avoid copper contamination of the cast strand surface. Thermal and mechanical strains cause a distortion of the mold and thus affect product quality. Thermal strain distortions, which are highest at the meniscus level where the steel temperature is highest in the mold, cause permanent distortion due to lower yield strength of copper. Mold wear at the exit end also causes reverse taper phenomenon (Ref 44). Heat Transfer in Continuous Casting Heat transfer conditions in the mold have been thoroughly investigated through modeling and plant verification (Ref 45). The predominant transverse heat transfer can be considered as a flow of heat energy through a series of thermal resistances from the liquid steel at the core and the water sink of the cooling system. Heat transfers (a) in the solidifying casting, (b) from the steel shell surface to inner copper lining, (c) through the copper lining and (d) from the outer copper lining to the mold-cooling water. Sensible heat changes (the heat absorbed or evolved by a substance during a change of temperature that is not accomplished by a change of state) in the steel strand due to lowering of temperature and latent heat release due to phase changes comprise the heat to be transferred in the solidifying casting. In addition, a mushy zone exists between the liquid and the solid shell, the thickness of which is dependent on the steel carbon level. Also, the thickness of the solid shell changes continuously from the meniscus to the bottom of the mold, and transfer through this shell is by conduction. Transfer of heat from the steel shell to the mold wall is complex and occurs by radiation as well as conduction due to the tendency of the formation of an air gap between the mold and the shrinking cast shell. A large air gap, which can form longitudinally as well as in transverse direction, represents the slowest of the heat transfer steps and therefore, controls the overall rate of heat transfer. While bulging due to internal ferrostatic pressure tries to reduce the air gap, an increase in shell thickness away from the meniscus tends to resist bulging. Thus, the formation of the air gap is a dynamic phenomenon. Because mold taper is intended to reduce the air gap, it enhances heat transfer. At the inner copper wall, heat transfer is also affected by the quantity and type of mold flux and lubricant used. Fluxes and oil that wet copper assist in heat transfer. In general, the local heat flux down the mold length is maximum just below the meniscus and gradually decreases down the mold length. The average heat flux for the entire mold increases with increasing casting [...]... 0 .30 >0.4 0-0 .08 incl 0. 03 >0.0 8-0 . 13 incl Phosphorus 0.20 0.05 >0.05 0-0 .09 incl 0. 03 >0.0 9-0 .15 incl 0.05 >0.1 5-0 . 23 incl 0.07 >0.2 3- 0 .35 incl 0.09 Silicon (for bars)(b)(c) 0.08 0.15 >0.1 5-0 .20 incl 0.10 >0.2 0-0 .30 incl 0.15 >0 .3 0-0 .60 incl 0.20 Copper When copper is required, 0.20 minimum is commonly used Lead(d) When lead is required, a range of 0.1 5-0 .35 is generally used Incl, inclusive Boron-treated... low-carbon contents or through low-to-high alloy additions in combination with medium-to-high carbon levels (>0.40% C) The former type of high-alloy steels with low carbon are used for turbine blades, rings, bolts, and casings, whereas the latter medium-to-high, carbon-bearing steels are needed for high-strength rail steels, bearing steels, forging steels, tool steels, high-speed steels, and high-carbon... 2, 3, 4, and 5 Table 2 Carbon steel cast or heat chemical limits and ranges Applicable only to semifinished products for forging, hot-rolled and cold-finished bars, wire rods, and seamless tubing Element Carbon(a) Maximum specified element, % 0.12 of Range, % >0.1 2-0 .25 incl >0.2 5-0 .40 incl 0.06 >0.4 0-0 .55 incl 0.07 >0.5 5-0 .80 incl 0.10 >0.80 Manganese 0.05 0. 13 0.40 0.15 >0.4 0-0 .50 incl >0.5 0-1 .65... machinability (c) When silicon is required for rods the following ranges and limits are commonly used: 0.10 max; 0.0 7-0 .15, 0.1 0-0 .20, 0.1 5-0 .35 , 0.2 0-0 .40, or 0 .3 0-0 .60 (d) Lead is reported only as a range of 0.1 5-0 .35 % because it is usually added to the mold or ladle stream as the steel is poured Table 3 Carbon steel cast or heat chemical limits and ranges Applicable only to structural shapes, plates, strip,... by Air-Water Mist Cooling in Strand Casting, Continuous Casting, Vol 2, Heat Flow, Solidification and Crack Formation, ISS of AIME, 1984, p 13 3- 1 38 47 N.A Shah and J.J Moore, A Review of the Effects of Electromagnetic Stirring (EMS) in Continuously Cast Steels, Continuous Casting, Vol 3, The Application of Electromagnetic Stirring (EMS) in the Continuous Casting of Steel, ISS of AIME, 1984, p 3 5-4 6 48... Applicable only to structural shapes, plates, strip, sheets, and welding tubing Element Carbon(a)(b) Maximum specified element, % 0.15 of Range, % 0.05 >0.1 5-0 .30 incl 0.06 >0 .3 0-0 .40 incl 0.07 >0.4 0-0 .60 incl 0.08 >0.6 0-0 .80 incl 0.11 >0.8 0-1 .35 incl 0.14 ... mill capacity and capability Common types of primary mills include two-high reversing mills, two-high tandem mills, three-high mills, three-high billet mills, cross-country billet mills, and various combinations Two-high reversing mills are most versatile in their capability to roll different sizes of ingot (As the name implies, a two-high stand consists of two rolls, one above the other.) The rotation... processing low-carbon strip steel Interstitial-Free Steels Flat-rolled steels with carbon less than 0.0 03% and manganese below 0.18%, with special emphasis on low nitrogen to prevent aging effects and low hydrogen to prevent flaking, are classified as interstitial-free steel Lowering of carbon and nitrogen is achieved by niobium, titanium, and boron treatments during secondary steelmaking A boron-to-nitrogen... nonmetallic inclusions Mold fluxes melt and attain optimal fluidity when they come in contact with the molten steel Solid mold fluxes are SiO2-CaO-Al2O3-Na2O-CaF2-based with small additions of carbon Iron oxide is avoided to prevent reoxidation Fluxes are either fly-ash based or synthetically produced as frits or granulated powder Cooling Characteristics Secondary cooling, strand containment, and a withdrawal... Structural quality Cold-drawing quality Cold-pressing quality Cold-flanging quality Forging quality Pressure vessel quality Hot-rolled carbon steel bars Merchant quality Special quality Special hardenability Special internal soundness Nonmetallic inclusion requirement Special surface Scrapless nut quality Axle shaft quality Cold extrusion quality Cold-heading and cold-forging quality Cold-finished carbon . heat is decarburized in the AOD vessel to 0. 03% C in stages during which the inert gas to oxygen ratio of the blown gas increases from 1-to -3 to 3- to-1. During the blowing, fluxes are added to. Solid mold fluxes are SiO 2 -CaO-Al 2 O 3 -Na 2 O-CaF 2 -based with small additions of carbon. Iron oxide is avoided to prevent reoxidation. Fluxes are either fly-ash based or synthetically. graphite. Figure 21 shows an example of a ladle-to-tundish and tundish-to- mold refractory-tube shrouding system. Fig. 21 Schematic of a refractory-tube shrouding system for minimizing oxidation