Continuous Casting

(In continuous casting, metal is continually poured from a large, replenishable melt reservoir into an open, chilled mold, where it solidifies.

From: Engineering Materials Science , 1995

Continuous Casting

B.G. Thomas , in Encyclopedia of Materials: Science and Technology, 2001

1 Steel Continuous Casting

Continuous casting is a relatively new process in historical terms. Although the continuous strip casting process was conceived by Bessemer in 1858, 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 mold, tears, and allows molten steel to pour out over the bottom of the machine. This problem was overcome by Junghans in 1934 by vertically oscillating the mold, utilizing the concept of "negative strip" where the mold travels downward faster than the steel shell during some portion of the oscillation cycle to dislodge any sticking ( Wolf 1992). Many other developments and innovations have transformed the continuous casting process into the sophisticated process currently used to produce over 90% of steel in the world today, including plain carbon, alloy, and stainless steel grades (Wolf 1992).

The continuous casting process for steel is shown in Fig. 1 (second frame) and the close-up of the upper mold region is shown in Fig. 2. In this process, molten steel flows from a ladle, through a tundish into the mold. The tundish holds enough metal to provide a continuous flow to the mold, even during an exchange of ladles, which are supplied periodically from the 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, surface defects such as "slivers" may form during subsequent rolling operations; or local internal stress concentration may occur, leading to a reduction in fatigue life. To produce higher quality product, the liquid steel must 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 will react to form detrimental oxide inclusions in the steel.

Figure 2. Schematic of mold region of continuous casting process for steel slabs.

Once in the mold, the molten steel freezes against the water-cooled walls of a bottomless copper mold to form a solid shell. The mold is oscillated vertically in order to discourage sticking of the shell to the mold walls. Drive rolls lower in the machine continuously withdraw the shell from the mold at a rate or "casting speed" that matches the flow of incoming metal, so the process ideally runs in steady state. 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 mold.

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 mold 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 mold slag is added to the steel meniscus, which flows into the gap between the mold and shell. In addition to lubricating the contact, a mold slag layer protects the steel from air, provides thermal insulation, and absorbs inclusions.

Below the mold exit, the thin solidified shell (6– 20   mm thick) 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 molten core is solid. After the center is completely solid (at the "metallurgical length" of the caster, which is 10–40   m), the strand is cut with oxyacetylene torches into slabs or billets of any desired length.

Different continuous casting processes exist to produce cross-sections of different shapes and sizes. Heavy, four-piece plate molds with rigid backing plates are used to cast large, rectangular "slabs" (50–250   mm thick and 0.5–2.2   m wide), which are rolled into plate or sheet. Similar molds are used for casting relatively square "blooms," which range up to 400×600   mm in cross-section. Single-piece tube molds are used to cast small, square "billets" (100–200   mm thick) which are rolled into long products, such as bars, angles, rails, nails, and axles. A new strip casting process is being developed using large rotating rolls as the mold walls to solidify 1–3   mm thick steel sheet.

When casting large cross-sections, such as slabs, a series of rolls must support the soft steel shell between the mold exit and the metallurgical length, in order to minimize bulging due to 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 shown in Fig. 1. If the roll support and alignment are not sufficient, internal cracks and segregation may result. These defects will persist in the final product, even after many rolling and other operations, so it is important to control the casting process.

The process is started by plugging the bottom of the mold with a "dummy bar." After enough metal 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 molten steel supply is stopped and the process must be restarted. The maximum casting speed of 1–8   mmin−1 is governed by the allowable length of the liquid core and to avoid quality problems, which are generally worse at higher speeds.

After the steel leaves the caster, it is reheated to a uniform temperature and rolled into sheet, bars, rails, and other shapes. Modern steel plants position the rolling operations close to the caster to save on reheating energy. Further information on the continuous casting of steel can be found elsewhere (Wolf 1992, Schrewe 1991, Irving 1993, Cramb 1991, 2001, Continuous casting 1979–1997). The application of computational models to understand and improve this process is discussed elsewhere (see Continuous Casting: Complex Models ).

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Advanced Fuels/Fuel Cladding/Nuclear Fuel Performance Modeling and Simulation

T. Ogata , in Comprehensive Nuclear Materials, 2012

3.01.3.1.2.2 Continuous casting

Continuous casting is widely used in steel plants, and is also one of the candidates for MA-bearing metal fuel slugs. This process eliminates the need to use molds. KAERI produced a uranium rod with a uniform diameter of 13.7  mm and a length of 2.3   m. 57 The continuous casting of U–Zr alloy slugs with a smaller diameter is under way.

Optimizing the casting conditions is difficult when the fuel alloy has a large solidification range 52,57 (temperature difference between the solidus and the liquidus). A wide solidification range can lead to microshrinkage effects and loss of process control during casting. 52 Furthermore, pulling of the cast must be properly aligned to avoid any asymmetric variations in the rod diameter, thereby increasing the complexity of the unit for remote operation. 52 Finally, if continuous casting were to be used, the process would need to be highly automated to minimize the extent of human interaction required for casting a significant number of fuel slugs. 52

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Continuous Casting of Steel

Seppo Louhenkilpi , in Treatise on Process Metallurgy: Industrial Processes, 2014

1.8.1 Introduction

Continuous casting is the important linking process between steelmaking and rolling. As early as 1856, Henry Bessemer suggested a continuous casting method but just during the 1930s and 1940s continuous casting became a common production method for nonferrous metals and later from the 1960s for steels. The relatively low thermal conductivity of steel and the high casting temperatures meant that many problems had to be solved compared to nonferrous casting. In the mid-1980s, continuous casting grew into the biggest casting method, exceeding the conventional ingot steel casting route. In the ingot casting route, individual molds are filled with molten steel to produce steel ingots. The continuous casting method has a lot of benefits compared to the older ingot casting methods. The major advantages are improvement of steel quality, better yield, and savings of energy and manpower. Today, about 95% of the world's steel production is made by continuous casting and a great number of steel qualities are cast in very wide variety of dimensions.

The principle of the continuous casting method is simple (Figure 1.8.1). The liquid steel in a ladle is transferred to the casting machine. When the casting operation starts, the nozzle at the bottom of the ladle is opened and the steel flows at a controlled rate into the tundish and from the tundish through a submerged entry nozzle (SEN) into one mold or several molds. The molds are generally water-cooled copper molds. The first solidification takes place at the metal/mold interface. The thickness of the solidified shell increases progressively when it is withdrawn through the machine. At the mold exit, the shell must be thick enough to support the liquid pool. Below the mold, the shell is cooled by spraying water. The mold cooling is called the primary cooling and the spray cooling the secondary cooling. At the machine end, the strand is cut off and transferred to a rolling mill.

Figure 1.8.1. A schematic representation of the one strand, curved continuous casting process.

The big challenge in continuous casting is to cast steel continuously without interruptions and without many kinds of defects. Solidification control is important for surface and internal quality. Steel cleanliness is determined essentially already by the preceding operations in the ladle and in the tundish but can be influenced even in the casting operation. Important control parameters in solidification are, e.g., steel chemistry, casting speed, mold level, mold powder, mold oscillation, liquid steel temperature, secondary cooling conditions, as well as parameters affecting the flow phenomena in the mold. The research and development work in the continuous casting field is continuing quite intensively today, the main purposes being better quality of cast product and to develop methods to cast extra difficult steel grades with special problems and requirements. Today also the energy efficiency and the ecological aspects are of special importance.

Secondary steelmaking and continuous casting are the central process phases with strong influence on the final quality of the steel products. Liquid steel processing in ladle, tundish, and mold and final solidification consist of a complicated series of successive chemical, physical, and thermal phenomena. Strict control and smooth operation are extremely important but quite challenging tasks due to scarcity of direct measurements which would describe dynamic changes in steel chemistry, temperature, flow conditions, and interactions with, e.g., covering slag, refractory materials, or mold wall. Modeling of reactions, flow dynamics, and heat transfer can give a better understanding of different phenomena and their relations to different process parameters as well as it can advice to optimize the process run.

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CFD Case Studies

Jiyuan Tu , ... Chaoqun Liu , in Computational Fluid Dynamics (Third Edition), 2018

e1.3.8.1 Case Introduction

Continuous casting (CC) has been widely accepted as the most important production process in the steel industry. In the casting process, molten steel from the ladle flows through the tundish into a mould. Within the mould, the molten steel freezes against the water-cooled copper mould walls forming a solid shell. Argon gas is injected into the molten steel that enters into the continuous casting mould through the submerged entry nozzle (SEN). After the SEN, due to intense shear forces exerted by molten steel, the argon gas disintegrates into swarm of bubbles with different diameters. As illustrated in Fig. e1.68, large bubbles have the tendency to escape from the liquid steel surface through the mould flux power layer, whilst smaller bubbles follow the main stream of molten steel flowing deep into the mould cavity. However, these small bubbles and non-metallic inclusions adhering to the surface of these bubbles may be entrapped by solidified shell, forming defects in the final product, such as slivers, blisters, and other costly defects. In order to improve the quality of the final steel products, it is crucial to gain an in-depth understanding of the structure characteristics of molten steel-argon gas two-phase flows and the characteristics of bubble-size distribution and its related defects in the current continuous casting process.

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Manufacturing methods

Donald B. Richardson , ... (Section 16.5), in Mechanical Engineer's Reference Book (Twelfth Edition), 1994

16.5.3.3 Continuous casting of aluminium

When continuous casting of aluminium alloys was first introduced it provided a very significant improvement in soundness and fineness of structure compared with the 'can cast' method of ingot production, which was the best previously available. A very large number of systems - some vertical, some horizontal, some utilizing fixed moulds while some depend on casting wheels, bands, segmented moulds or rolls - are summarized in Table 16.16. 91

Table 16.16. Aluminium continuous casting systems

System Other alloys cast
Ingot, slab, and billet
Clark single-strand horizontal-casting system
Wagstaff horizontal-casting machine
Reynolds horizontal-casting process
Kaiser aluminium process for horizontal continuous casting
Alcoa horizontal continuous-casting process
Sheet, plate, and foil
Hunter continuous-casting process
Pechiney 3C process
Mann rotary strip casting and rolling line
Alusuisse caster I and caster II
Hazelett twin-belt caster Zn, Pb, Cu Steel
Rod, bar, and wire
Properzi process for continuously cast and rolled rod Pb, Zn, Cu
Cegedur-Pechiney-Secim continuous casting and rolling process
Southwire aluminium SCR systems Cu

Table 16.17 91 places the systems in approximate order of cooling rate and the fineness of structure of the material they produce. The twin-roll and stationary mould strip casters will produce 3–7 m dendritic structures in 12.5 mm thick strip.

Table 16.17. Classification of commercial and experimental casters for aluminium alloys

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Crystalline Silicon

Francesca Ferrazza , in McEvoy's Handbook of Photovoltaics (Third Edition), 2018

4.3 Electromagnetic continuous casting

Electromagnetic continuous casting (EMC) uses an RF coil to induce currents in an appropriately designed circuit able to push the melt away from the walls, therefore making it unnecessary to use crucibles. A schematic of the furnace is shown in Fig. 5 [19]. The process is carried out in argon ambient at slight overpressure. The top end is open for the ingot to be pulled down while new feed material is added. The resulting ingot is a long bar of about 240   kg in weight. The idea behind this growth method is to completely avoid the use of any physical crucible by confining the charge electromagnetically. This gets rid, at one time, of two major issues of the DS techniques that have previously been described: expensive crucibles and related contamination. However, inhomogeneous nucleation occurs, and grain size is also rather small, resulting in a low-starting quality of the material [20], although a low-oxygen content is reported. This kind of material is not commercially available yet, but there are announcements that it could be shortly.

Figure 5. Schematic of Electromagnetic continuous casting (EMC) furnace.

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Recent Progress in the Continuous Casting of Steel

A.W. Cramb , in Encyclopedia of Materials: Science and Technology, 2004

3 Conclusion

Even though conventional continuous casting operations are becoming mature technologies, there are still significant advances under development that may significantly improve either quality and productivity or the application range of the technology. Both of the above developments will in time lead to significant improvements in continuous casting and will lead to both future and current casting technologies being significantly more technology focused in the past. In addition to these developments, significant effort has been expended on advanced control systems based on multiple sensor input arrays to allow optimized operation with on-line quality prediction. Thus, continuous casting machines have become sophisticated almost-robotic operations, with future trends being towards full automatic control with no manual intervention.

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How Mold Fluxes Work

Ken Mills , in Treatise on Process Metallurgy: Industrial Processes, 2014

Abstract

The continuous casting of steel is a very successful process. One of the factors underlying this success is the performance of the mould flux. The mould powder is expected to be sufficiently " flexible or forgiving" to accommodate changes in casting conditions. However, it is necessary to select the mould powder for the given casting conditions in order to minimise process problems and product defects. This entails a detailed knowledge of how the casting powder behaves in the mold where it, successively, (i) sinters, (ii) melts to form a slag pool whereupon and (iii) molten slag infiltrates into the mould / strand gap to form solid and liquid slag films. The various factors affecting each of these processes are identified; this allows the factors to be manipulated to optimise the feature being studied e.g. the depth of the slag pool.

The key functions of the mold slag are the lubrication, represented by the powder consumption, and the horizontal heat flux. These are, in turn, related to the thicknesses of the liquid and solid slag films formed between the steel shell and the mold. The various factors e.g. casting speed and slag properties, affecting both the powder consumption and the heat flux are reviewed and evaluated. The effect of various casting variables like casting speed and oscillation characteristics on casting flux performance are also reviewed.

Information on the key properties like slag viscosity affecting mould powder performance is provided. The rules for optimum selection of the key properties (viscosity, break temperature etc.) for the given casting conditions are summarised. Finally, the chapter contains a summary of (i) the causes of product defects and process problems, (ii) the strategies used to counter them and (iii) suggested remedial treatments for the steelmaker to use.

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Solidification and steel casting

A.W. Cramb , in Fundamentals of Metallurgy, 2005

10.4.3 The surface of steel cast continuously in an oscillating mold

During the continuous casting of steel it is common to see surface marks on the surface of cast material. Generally there is at least one mark per oscillation cycle of the mold. This is shown in Fig. 10.27 where the steel surface is shown on the right-hand side of the picture (b) and the slag surface sticking to the mold is shown on the left-hand side of the picture (a). Thus, in operation, both sides would be in contact and the slag surface would be an exact fit to the steel surface. 30

The surface topography of continuous cast steels is dependent upon the thermal conditions in the mold and the steel grade that is cast (Fig. 10.31). In general ultra low carbon steels, which tend to have very low solute content, exhibit deep marks that also form hook structures in the surface of the cast slab where it appears that the liquid meniscus solidified and then was subsequently overflowed. In peritectic steels there are also deep marks but the steel surface itself is also wrinkled where in medium carbon grades the oscillation marks are very small and are more like undulations (Fig. 10.32).

10.31. The surface of continuous cast steel: (a) is the mold slag sticking to the mold and (b) is the surface of the steel. 27

Photographs by A. Badri.

10.32. Profile of oscillation marks as a function of steel grade. 30

In the formation of oscillation marks in ultra low carbon steels, where hook formation is common, recent work has shown that in this case the mark is formed by an increased rate of heat transfer during the negative strip period of mold oscillation due to the liquid meniscus moving closer to the mold wall. Negative strip time is defined as the period during which the mold is moving downwards faster than the strand, while the remaining duration of the oscillation cycle is called the positive strip period. For sinusoidal oscillation, negative strip time is quantified by the following equation:

(10.87) t n = 1 π f arccos v c π s f

where tn is the negative strip period, f is the frequency of oscillation (Hz), vc is the casting speed, and s is the stroke.

Thermal measurements and heat flux calculations by Badri et al. have documented this increased heat transfer rate during the negative strip time of mold oscillation (Fig. 10.33) and have shown that local changes in meniscus position during the negative strip time that give rise to increased heat transfer rates are one mechanism of mark formation. 30–32 Details of this mechanism are shown schematically for an overflow type of mark in Fig. 10.34.

10.33. Variation of heat flux about an average value during the continuous casting of an ultra-low carbon steel (horizontal bars are the negative strip time). 30

10.34. Schematic of the formation of an overflow oscillation mark. 30

In Badri's work, 30 there are two necessary conditions for the formation of a solidified meniscus. First, the mold conditions at the meniscus must be such that the potential for heat transfer is sufficient to cause solidification of the curved meniscus. Second, the liquid meniscus must deform such that the liquid comes into close proximity with the copper mold. The simultaneous occurrence of these two necessary conditions provides the sufficient condition for the solidification of the meniscus. The final necessary condition determines the type of mark that forms. The frozen meniscus can be overflowed to form a subsurface hook-type oscillation mark, or, if the frozen meniscus lacks strength, the rising liquid can force the shell back to the mold, forming a depression-type mark. Therefore, the first two necessary conditions must occur simultaneously, followed by the third condition, to create a series of events necessary and sufficient for the formation of oscillation marks (Fig. 10.34). It should be noted that meniscus movement can be caused by fluid flow as well as mold oscillation and the extra marks that are often seen in continuous cast surfaces are often due to loss of level control and wave motion in the mold of the continuous caster.

During the negative strip period, where there is little relative motion between the mold and the shell, undercooled growth of dendrites combined with normal solidification might be expected and dendrites will grow along the meniscus as noted first by Saucedo. 33

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Modern Ironmaking and Steelmaking

Vaclav Smil , in Still the Iron Age, 2016

Continuous Casting

Post-WW II changes in the treatment of steel after its production have been no less important than was the shift to BOFs and EAFs. Long-standing practice was inherently energy intensive as the hot metal was first cast into steel ingots, oblong pieces that could weigh between 50   t (for specialty steels) and 500   t (for steel destined to be forged into large pieces) and had to be reheated before they could go through a primary rolling mill, where they were formed into one of the three basic kinds of semifinished shapes: wide (some more than 3   m) and thick (up to 25   cm) slabs; square-profile (up to 25   cm) billets; and rectangular-profile blooms (commonly 40 ×60   cm). Final processing (hot- or cold-rolling) turned standard- and medium-thickness slabs into thin slabs and plates, billets into bars and rods, and blooms into I and H beams. The complete casting and rolling sequence could require no less energy than the steelmaking itself, but it persisted for generations, being the final component of the traditional combination that began with blast furnace pig iron and continued with open hearth steelmaking.

Steps eliminated by continuous casting include the pouring of hot metal into ingot molds, removing the molds from the ingots, putting the ingots into soaking pits in order to equalize their temperature, and then rolling them into semifinished products (slabs, billets, or booms). Much like the idea of the oxygen furnace, the concept of continuous casting had also originated with Henry Bessemer: his first patent for a twin-roll caster (using water-cooled rolls with edges sealed by a flange or a dam in groove) was granted in 1865, and the original design was improved in 1891 ( Bessemer, 1891), a couple years after R.M. Daelen received a German patent for a vertical casting machine. But Bessemer's peers were unimpressed:

One need not, therefore, be greatly surprised that the production of continuous sheets direct from fluid iron did not excite a great amount of enthusiasm in the minds of tin plate manufacturers of that day; in fact, the whole scheme was simply pooh-poohed and laid aside, without any serious consideration of its merits

(Bessemer, 1891, p. 27).

Problems with metal sticking and uneven cooling prevented the conversion of these early designs into functioning machinery, and the first successful commercial applications of continuous casting, during the 1930s, were not for steel but for the castings of color metals with significantly lower melting points. Continuous casting of steel owes it eventual success to the combination of the metallurgical expertise of Siegfried Junghans (1887–1954) and entrepreneurial effort of Irving Rossi (1889–1991). A key to this advance was the invention of a vertically oscillating (reciprocating) mold by Siegfried Junghans: it eliminated the possibility of the cooling metal sticking to the mold.

The first working prototype was ready in 1927; Junghans filed a patent claim in 1933 (inexplicably, it was not granted until 1944) and afterwards devoted himself to adapting the process for steel casting. When, in 1936, he demonstrated his technique with brass casting to Irving Rossi, an American engineer doing business in pre-WW II Germany, it had actually ended in failure as the brass billet skin tore open and hot metal spewed out. But Rossi had correctly distinguished between the challenges with a prototype and the far-reaching commercial potential of the demonstrated technique and immediately secured exclusive rights to Junghans' patent and to its follow-ups for the United States and England and nonexclusive rights for all countries outside of Germany, and in 1938 he added an agreement for sharing information required to build new continuous casting plants in return for financing such developments outside of Germany (Tanner, 1998).

Rossi's first American commercial installation came in 1937 with German-built brass casting, but the need for rapid and massive expansion of wartime steelmaking favored the use of well-established methods, and so it wasn't until 1947 that Rossi persuaded Alleghany Ludlum Steel to embrace the method. The first good-quality slabs (33 ×7.5   cm, and up to 10   m long) were made in May 1949 in the company's Watervliet plant by an American-made (Koppers) casting machine, but this early attempt did not lead to a further commercial adoption. Junghans made his first successful continuous steel casting also in 1949 in his workshop, and in 1950 Mannesmann (the other owner of basic patent rights) acquired the rights and began to build the first German commercial line at Huckingen, and it began operation in 1952. Rossi expanded his promotion and licensing activities by establishing Continuous Metalcast Corporation and, in October 1954, Concast AG in Zurich. Its general manager, Swiss lawyer Heinrich Tanner, eventually wrote a detailed definitive history of the early era of this fundamental technical innovation (Tanner, 1998).

During the next three decades, Concast dominated the global expansion of continuous casting through two highly profitable arrangements. In return for licensing its key continuous casting patents the company first received, gratis, all information and patents arising from the operation of licensed plants. This arrangement was further strengthened in 1970 when Concast made a cooperative patent exchange agreement with Mannesmann, its principal competitor. Second, builders of casting machinery channeled their sales contracts solely through Concast, receiving in return worldwide marketing, and consulting services guaranteed to resolve any technical problems arising during the early phases of commercial operation. As a result, Concast had eventually controlled more than 60% of the global market for continuous casters, it was identified as a virtual monopoly, and in 1981 it had to be reorganized pursuant to antitrust rulings in the United States and Europe.

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