Continuous casting is a continuous casting process used to manufacture semi-finished products from ferrous and non-ferrous alloys . In continuous casting, the metal is poured through a cooled bottomless mold and pulled downwards, sideways or upwards with the solidified shell and mostly still liquid core. After solidification, the strand is divided.
According to the mold arrangement , a distinction is made between horizontal and vertical continuous casting. Horizontal continuous casting is only used for non-ferrous metals. Vertical continuous casters with a curved mold cast a pre-bent semi-finished product so that it can be drawn off horizontally. The semi-finished product must then be straightened after it has reached the horizontal, before cutting. With vertical molds, either the length of the casting is limited by the height of the system, or the strand has to be bent with the core still liquid and straightened again when it is horizontal.
For the vertical continuous casting, which is mainly used for non-ferrous metals, a water-cooled copper mold, which is open at the bottom, is used, also called a collar mold. Liquid metal is fed to it via a distribution system, continuously at a metered casting speed. In the simplest form, there is a pouring crucible, also called a tundish, between the furnace and the mold. A strand shell is formed inside the mold, enclosing the initially still liquid core. The strand emerging from the mold is lowered into a basin that promotes further cooling and is constantly sprayed with water. In the discontinuous process, the depth of this cooling basin limits the length of the billets, bars or blanks.
Continuous casting plants are built with one, two or more strands. The smaller the cross-sections, the more strings are required to meet the capacities of the upstream units. In contrast, mold systems are used, especially when casting round bars (also known as circular blanks or extrusion billets), which enable up to 32 circular blanks with diameters in the range of 120–150 mm to be cast at the same time.
The technology of continuous casting differs little, whether steels , copper alloys or aluminum are processed. The main difference lies in the temperatures, which range from approx. 700 ° C for pure or alloyed aluminum to> 1600 ° C for steel. Attempts to cast continuously go back to the mid-19th century. Decisive progress was made from 1930 (Junghans-Rossi process for light and heavy metals).
In contrast to permanent mold casting, which produces individually shaped castings, continuous casting is a semi-continuous to continuous process. In the case of semicontinuously operating systems, their height determines the maximum length of the respective continuously cast product. Continuous work means casting an endless strand. This can either be divided by a saw ("flying saw") as soon as a strand section has solidified sufficiently, or the strand is diverted to the curve and leaves the system as a horizontal strand. This technology is used, among other things, in the manufacture of continuous casting pigs for foundries, or in the processing of copper materials into bar material or pipes. Continuous castings made from copper alloys are also cast both horizontally and vertically.
The continuously cast products are called semi-finished products because they have to undergo further processing steps up to the end product. Rolling , pressing and deep drawing are mostly used, combined with a prior time-scheduled outsourcing for space or Ambient temperature, or an upstream or downstream thermal treatment (tempering and aging).
Continuous casting of steel
A continuous caster consists of
- the ladle turret for inserting the steel ladle,
- the distributor that directs the melt to the mold,
- the mold with the melt (primary cooling area),
- Immersion tube and shadow tube
- Slate plate to regulate the liquid steel
- the mold oscillation and holding device,
- the strand guide system (casting arch) with secondary cooling,
- the driving and straightening devices,
- the outfeed roller table,
- the cut-to-length line (flame cutting machine, shears),
- the marking machine (marking) and
- the cold strand.
The melt is usually transported with a so-called ferry from the melting unit via the casting crane to the continuous caster and used in the ladle turret. The melt flows into the distributor via a bottom outlet (spout) that is closed by a slide. So that the liquid steel does not react with the oxygen in the air, the liquid steel is guided in a shadow tube or covered with casting powder. The distributor initially performs the function of a buffer vessel so that the continuous casting process is not interrupted when the ladle is changed. In multi-line systems, the distributor also has the function of distributing the liquid steel to the individual lines. The melt is covered in the distributor by a layer of slag (mostly rice husk ash).
The melt runs freely from the distributor or through a dip tube (also known as a pouring tube) into the mold . The flow is controlled by a plug or a slide depending on the meniscus in the mold. In order to avoid caking of alumina (Al 2 O 3 ) on the distributor spout , it is partially flushed with argon and the melt can be braked or stirred electromagnetically in order to influence the flow of the liquid steel. The mold level in the mold is covered with slag. This prevents reoxidation of the melt, binds impurities that have risen and acts as a lubricant between the solidified shell and the mold. To form the slag, casting powder is continuously applied to the meniscus. During casting, the mold is moved in an oscillating manner to prevent the steel from sticking to the cooled walls of the mold (e.g. copper and nickel plates) and to support the transport process. When leaving the mold, the strand has a solidified shell a few centimeters thick, while the majority of the cross-section is still liquid. Below the mold, the strand is cooled from the outside in the so-called casting arc (circular zone radius 10 m) by spraying air / water (secondary). It is also supported by rollers to reduce the amount of bulging that occurs from the ferrostatic pressure on the strand shell . These strand guide rollers, which are exposed to the highest levels of stress, are among the central components of a continuous caster because of their influence on the strand surface. For this reason, the rollers are provided with a corrosion- and wear-resistant layer that was previously welded on. Today these roles are more and more often produced as "composite casting" using the centrifugal casting process.
Compared to other metals such as aluminum or copper, steel has a relatively low thermal conductivity . This results in a long strand length of up to 20 m from the meniscus to the complete solidification of the melt (so-called “metallurgical length”). Only after solidification can the strand be cut into individual slabs . Systems in which the strands are continued vertically after leaving the mold therefore require a large overall height and are only rarely used. In most systems, the strand is bent in a radius (depending on the strand thickness) of around 7 to 15 m, until it is straightened again with a bending and straightening unit when a horizontal angle is reached. The mold can either be straight or curved. In straight molds, the strand leaves the mold vertically downwards and is only bent by the subsequent roller guide; in curved molds, the strand is already circular. Both concepts have their specific advantages and disadvantages, which result from the fact that, on the one hand, cracks occur as a result of the bending of the strand and, on the other hand, a long vertical liquid path offers advantages for the purity of the steel, since impurities can rise into the slag. After solidification, the strand is cut into lengths on the discharge roller table with burners .
For casting on, a starting line (cold line) is introduced into the mold from below or above, which closes the underside. The steel filled in solidifies on the starting line and is pulled down with it. After the metallurgical length has been reached, the start-up strand and the cast strand are separated from each other (uncoupled).
Process types according to the end product
In continuous casting, a distinction is made between several processes, which depend on the format of the strand to be cast:
- In continuous billet and bloom casting , round or nearly square cross-sections ( billets ) or even profile-shaped cross-sections are cast. They are used to manufacture rods, wires and profiles.
- In continuous slab casting, slabs, i.e. rectangular strands with a large width (up to over 2600 mm) and small thickness (up to 600 mm), are produced for sheet metal production . A further distinction is made in continuous slab casting:
- Continuous slab casting in general (thickness over 100 mm to 600 mm)
- Slab casting or (thickness from 70 to 300 mm)
- Thin slab casting (thickness from 40 to 100 mm)
- Pre-strip casting (thickness from 15 to 50 mm)
- Strip casting (thickness a few mm).
With the last two methods mentioned, considerable savings in the area of system technology and the energy balance are possible. A further reduction in the forming processes is achieved by thin strip casting (see there).
The pre-strip casting produces a thin slab with a thickness of around 50 mm, which can be pierced directly in the production line of a hot rolling mill. Only one equalizing furnace is required between the casting machine and the rolling unit. With this method many steel qualities can be processed inexpensively. Disadvantages are the low flexibility and surface quality of the strips.
Strip casting is a continuous casting process with which metal strips are produced, which are then either not formed or only formed in a few passes (passing through a roll stand ). The process is mainly used for non-ferrous metals.
The 2-roll belt casting of steel was patented by Henry Bessemer in the 19th century . However, he was unable to make it ready for the market. It was only used industrially for stainless steels and electrical steels after 1990. Here the steel solidifies between two counter-rotating water-cooled rollers and is completely solidified as a band with a max. 6 mm thick, discharged downwards. The speed of the casting rollers must be precisely matched to the casting temperature, since a breakthrough occurs if the solidification is not complete (casting temperature and / or roller speed is too high). However, if the melt solidifies too early, the forces on the rollers are too great and can even paralyze the process (casting temperature and / or roller speed too low).
A rolling stand is typically integrated into the system, in which (with a low casting thickness) a strip thickness of 1 mm - which is also the possible minimum - results after a single rolling pass. The rolled thin strip is water-cooled and wound onto rolls, so-called “ coils ”.
The process offers the advantage of low investment requirements and the possibility of producing smaller lots at low cost. Since the reheating required for rolling the slabs is no longer necessary, there are further savings in terms of the cost factors of time and energy.
Continuous casting of aluminum
Hilmar R. Müller gives an almost complete overview of the continuous casting processes for non-ferrous metals: Continuous casting of non-ferrous metals - an overview . In: Material Week 2015 in Dresden . Symposium 18 - Continuous casting of non-ferrous metals, doi : 10.13140 / RG.2.1.1251.3762 / 1 . .
. (2015). (Continuous Casting of Non-ferrous Metals - an Overview).
The continuous casting of aluminum, also under the overall designation formats cast driven, as well as other as a batch or continuous process to be carried out. In the first case, bars or round blanks of the length specified by the system are produced, in the other case the cast strand is cut up by a flying saw after reaching a certain length, as the technical term is used, and this as the casting process continues. The technology can also be described as semi-continuous, the aforementioned endless strand offers real continuity.
Regardless of such a system-related distinction, the liquid metal is also supplied to the mold in the case of aluminum via a pouring crucible, or via a pouring channel and a distribution system downstream of the mold, or several. In the casting channel, additives important for the alloy can be added - automatically dosed depending on the casting speed. Mostly it is master alloys containing titanium and boron to influence the structure ( grain refiners ). The externally rapidly solidified strand in the water-cooled mold, called a primary cooling process, is drawn off into a cooling basin (secondary cooling) for further cooling and solidification. Lowering and inflow in the mold take place simultaneously. The casting or lowering speed, which averages between 5 and 15 cm / min, is determined by the cast alloy.
Different solidification speeds of metal melts are always visible in the cast structure. Due to the very fast solidification caused by the water cooling, compared to sand casting, for example, there are no phase precipitations, that is, one normally does not find any differences in composition when examining strand cross-sections using spectral analysis spectrography . However, the rapid extraction of heat during continuous casting prevents tension within the solidification structure from being balanced. Cold aging or heat treatment (solution annealing) eliminate the stresses that can lead to cracking during rolling. Segregation-like exudations in the cast skin are not eliminated in this way; the cast skin is then milled off before further processing. The casting error can be limited if care is taken to always keep the alloy-dependent shrinkage gap between solidifying metal and mold wall as small as possible, since air hinders the dissipation of heat. A far greater problem than the shrinkage gap in the continuous casting of aluminum and its alloys is the sensitivity of the liquid metal with regard to hydrogen absorption and oxide formation , which each require special treatment of the melt.
Melt treatment in continuous casting
The problem of hydrogen uptake, avoidance and removal
If the continuous casting is produced in a foundry that is subordinate to a primary smelter - internationally technically speaking a cast house - this is only delivered to metal that has previously been melt- treated, be it as pure aluminum or already alloyed on the smelter side . The treatment, which takes place in a mixer or holding furnace, has the primary goal of removing impurities from the electrolysis from the melt and this means reducing the sodium and calcium content to a large extent. The removal of a given hydrogen content is done at the latest possible point. Before the turn of the century, chlorination by means of chlorine gas or chlorine-releasing additives was preferred because it quickly converted the two interfering elements into their chlorides and let them rise to slag. Excess aluminum chloride also had a useful flushing effect, as it exits the melt at given temperatures with boiling. For environmental reasons, the use of chlorine was increasingly reduced from around 1980 and argon or freon was used with only a small amount of chlorine. The invention of a SNIF box called the treatment chamber, which was connected directly upstream of the distribution system, enabled finer dosing and removal of the hydrogen at the last point before potting, thus limiting the possibility of resumption from humid ambient air. It is also state of the art to place a required grain refinement directly in front of the SNIF box with a controlled feed of a wire alloyed with fining elements. The removal of the oxides, unless they have already been rinsed out in the course of the treatment, a filtration (see there).
Recycling companies that can also produce continuously cast material from scrap have to do a little more, especially when it comes to removing hydrogen, since the melted scrap is often corroded, i.e. has a hydroxide layer or is oily. You should also pay more attention to interfering elements, as some of the recycling material comes from foundries and can contain numerous undesirable elements. Not all must or can be removed. With magnesium or copper contents, material suitable for continuous casting can be provided, silicon-rich melts are more suitable for the production of AlSi casting alloys.
The problem of oxide formation, prevention and removal
Given a given tendency to oxidation of aluminum melts, especially those with magnesium contents, it is necessary to limit the oxidation by applying inhibiting measures ( melt treatment ). Oxides floating in the melt and entering the casting, mostly gamma-oxides in the form of skins, always lead to casting defects. Protection against oxidation must also be ensured with casting technology. In the case of continuous casting, it can be done by transferring processes and there in the pouring stream, but also in the distribution system. Filtration offers good security according to the state of the art.
Oxide inclusions must be avoided in the manufacture of aluminum products that are subject to special quality requirements, be it for format casting (semi-finished products) or molded casting . If oxidation cannot be avoided, the melts must be cleaned by rinsing. In the past, this was achieved through a chlorinating treatment of the melt, i.e. the passage of gaseous chlorine, either from the bottle or released in situ. However, from today's perspective, the chlorination of aluminum melts has various disadvantages. Among other things, the magnesium content of the melt is reduced and, in particular, problems arise in terms of environmental protection , since some of the aluminum chloride formed and which is volatile at the melting temperature can get into the environment. The use of chlorine is also undesirable for workplace hygiene. Therefore, either inert gases or mixtures with low chlorine contents are used today. The degassing and at the same time flushing treatment, which reduces the hydrogen content, removes the coarser oxides which are contaminated with hydrogen. It can be supplemented by an additional filtration which also captures fine oxides.
Two filtration processes are considered state of the art: either using a porous ceramic filter or a filter bed through which the melt flows and which mostly consists of petroleum coke . Both processes are suitable for trapping oxides before the melt enters the mold.
A filter for metallurgical use that is called ceramic due to its material composition - not to be confused with frequency filters of the same name used in electrical engineering - can be both circular and square and, depending on the task at hand, has a certain number of pores per cm². In continuous casting, it is used as close as possible to the distribution system of the continuous caster and allows a certain amount of metal to flow through before it is replaced (disposable filter).
A notable advantage of filtration is the reduction in the reuptake of hydrogen associated with the oxide removal. In oxide-containing melts, hydrogen from humid ambient air can attach itself to the oxides present via the air-melt contact surface, forming hydroxides and leading to hydrogen contents which, when measured, are far above the solubility limit. As far as technically possible, protection of the bath surface with suitable salt mixtures is indicated, especially in the case of melts resting in holding furnaces, alternatively with a protective gas that is heavier than air.
- Schwerdtfeger: Metallurgy of continuous casting . Stahl Eisen Verlag, Düsseldorf 1992, ISBN 3-514-00350-5 .
- Aluminum paperback. Volume 1, 16th edition. Aluminum-Verlag, Düsseldorf 2002, ISBN 3-87017-274-6 .
- Dieter Altenpohl : Aluminum from the inside . 5th edition. Aluminum-Verlag, Düsseldorf 1994, ISBN 3-87017-235-5 .
- Stephan Hasse (Ed.): Foundry Lexicon. 17th edition. Verlag Schiele and Schön, Berlin, ISBN 3-7949-0606-3 .
- Melt treatment
- Material flow: melting, continuous casting, heat treatment, forging
- Photographs: ladle turret , tundish (distributor) , strand guide system (district zone)
- Almost 90% of all copper materials are processed continuously or discontinuously into semi-finished products in the continuous casting process. s. Brunhuber, Ernst, cast from copper alloys, translated from the American, Verlag Schiele & Schön, Berlin 1986, ISBN 3-7949-0444-3 .
- The world's first large-scale continuous casting plant for steel went into operation in 1948 at the Breitenfelder Edelstahlwerk in Styria .
- Technical note: To reduce the shrinkage gap, numerous measures have been proposed over the course of time, such as casting with the lowest possible metal level in the mold. Another possibility is the use of conical or grooved molds. A grooved mold causes air inclusions between the mold wall and the liquid metal to delay the dissipation of heat, so that the metal remains in contact with the cooling surface for longer with the same level of metal in the mold, but the contraction also shifts further downwards and the shrinkage gap becomes shorter. The contactless continuous casting in a magnetic field, invented in the former USSR, already became practical in the second half of the last century by the then Alusuisse , which largely eliminated the shrinkage gap problem.
- Technical note: The greater the water vapor pressure above the surface and the higher the temperature of the melt , the higher the hydrogen content, which is established as an equilibrium in the melt. The influence of temperature on the equilibrium solubility of hydrogen in aluminum is shown in Figure 1. Possible hydrogen contents of high-temperature melts are shown hatched in Figure 1. The need to reduce such contents is due to the fact that the hydrogen solubility of the aluminum after solidification falls to less than 10 of the solubility at liquidus temperature and the resulting excess shows more clearly as blistered cavities in the cast, the slower the solidification proceeds, e.g. with sand casting . In the case of very rapid solidification, as is the case with permanent metal molds, the risk of coarse-bubble precipitation is reduced; it is replaced by fine-grained bubbles or fine porosity. The hydrogen originally dissolved in the melt does not have enough time to escape from the rapidly solidifying structure of the melt, which means that a higher hydrogen content is usually determined in the solidified cast, for example by means of a density test , than soluble in the equilibrium state.
- A device that works on this basis is the Snif-Box , which enables melt cleaning in a "continuous process".