Continuous casting

Continuous casting or continuous casting - Product and Production Technology extent equal to putting - is called a method both for discontinuous as well as continuous production of ingots and billets, including blanks, made ​​of ferrous and non-ferrous alloys. In addition, the name rolling steel mill slab is still in use.

• 3.1 Special Technology 3.1.1 melt treatment in continuous casting 3.1.1.1 The problem of hydrogen uptake, prevention and removal
• 3.1.1.2 The problem of oxide formation, prevention and removal 3.1.1.2.1 melt filtration

Technology

The technique of continuous casting differs only slightly, whether steel, copper alloys or aluminum are processed. The main difference is at the temperatures ranging from about 700 ° C in pure or alloyed aluminum to> 1600 ° C for steel. Attempts to continuously pour, go back to the mid-19th century. Significant progress has been achieved since 1930 ( Junghans Rossi method for light and heavy metals).

Accordingly, the mold assembly is made between horizontal and vertical continuous casting, the former being applied specifically for copper alloys and small-sized strands. Einfachkokillen that generate only a square sheet ingot, are to large weights, or limited diameter of the round bar. Ingot weights of 30 and more tons are state of the art. (Also called blanks or billets ) particularly in the casting of billets, however, come Kokillensysteme used, which make it possible to pour up to 32 discs with diameters in the range of 120-150 mm at the same time.

In contrast to the individually shaped castings engendering chill casting Continuous casting is a semi-continuous to continuous process. In semi- continuous plants whose height determines the maximum length of each continuously cast product. Continuous work means casting a continuous strand. This can be divided by a saw ( "flying saw" ) either when a strand section is sufficiently solidified, or the strand is diverted towards the bow and leaves the system as a horizontal line. This technique is used among other things for the production of Stranggussmasseln for foundries, or in the processing of copper materials for bar stock or tubing. Even with continuous cast copper alloy is poured both horizontally and vertically.

Semi-finished products, the continuous cast products so named because they have to undergo to the final product still further machining operations. Rolling, pressing and deep drawing are mostly used, connected to a previous time terminated aging at room bezw. Ambient temperature, or in front of - and subsequent thermal treatment ( annealing and aging ).

For the mainly applied in non-ferrous metals vertical continuous casting an open bottom, water-cooled copper is used, also called Kragenkokille. Liquid metal it is continuously supplied via a distribution system at doses of the casting speed. In the simplest form this is between furnace and mold a Eingusstiegel, also called tundish. Within the mold, a strand shell that encloses the initially liquid core forms. The beam emerging from the mold string is lowered into the further cooling pool and conducive constantly sprayed with water. In the batch process, the depth limits this Abkühlbeckens the length of the billets, ingots or blanks.

Continuous casting of steel

Special technique

A continuous casting plant consists of

• The ladle turret for inserting the steel ladle,
• The distributor, who heads the melt to the mold,
• The mold with the melt ( primary cooling area)
• Kokillenoszillations and the holding device,
• The strand guide system ( casting arc ) with secondary cooling,
• The blowing and straightening apparatus,
• The run-out table,
• Length line ( cutting machine, scissors )
• Marking the ( signing) and
• The dummy bar.

The melt is usually transported with a so-called ferry from the melting unit on the casting crane to the casting plant and used in the ladle turret. About a floor drain ( sink ), which is closed by a slide, the melt flows into the manifold. Thus, the liquid steel does not react with oxygen in the air, the liquid steel is guided in a shade tube or covered with mold powder. The distributor initially fulfilled the function of a buffer vessel, so that it comes during a ladle change an interruption of the continuous casting process. In addition, the distributor has the function to distribute the liquid steel to the individual strands for multi- line systems. The melt is in the distributor through a slag layer ( usually rice husk ash ) covered.

From the manifold, the melt running free or with a dip tube (also called pouring tube ) into the mold. The flow is controlled in dependence on the meniscus in the mold with a stopper or a slide. To avoid caking on the manifold outlet cast by alumina (Al2O3), is partially purged with argon and the melt can be electromagnetically braked or stirred to influence the flow of the liquid steel. The meniscus in the mold is covered with slag. This prevents re-oxidation of the melt, binds Ascended impurities and serves as a lubricant between the solidified shell and the mold. To form the slag continuously casting powder is applied to the meniscus. During casting the mold is oscillated to prevent the steel from sticking to the cooled walls of the mold (for example, copper and nickel sheets ) and to assist in the transport process. On leaving the ingot mold the strand has a solidified shell of a few centimeters thick, while the bulk of the cross-section is still liquid. Below the mold, the strand ( 10 m radius circle zone) cooled in so-called casting sheet from the outside through Luft-/Wasserbesprühung (secondary). In addition, it is supported by rollers in order to reduce the amount of bulging, that occurs due to the forces acting on the slab shell ferrostatic pressure. This strand guide rollers that are subjected to high stresses include, as an influence on the strand surface of the key components of a continuous casting plant. For this reason, the rolls of a corrosion-and wear-resistant layer are provided, which has been welded before. Today, these roles are becoming more common as the " composite casting " produced by centrifugal casting.

Steel, in comparison with other metals such as aluminum or copper has a relatively low thermal conductivity. Characterized a large strand length up to 20 m from the meniscus resulting from the complete solidification of the melt (known as " metallurgical length "). After complete solidification of the strand can be cut into individual slabs. Systems in which the strands are further guided vertically after leaving the mold, thus requiring a large overall height and are rarely used. In most systems, the strand is bent in a radius (depending on the strip thickness ) of from about 7 to 15 meters until it is straightened, upon reaching a horizontal angle again with a bending and straightening unit. The mold can be either straight or curved run. In straight mold the strand leaves the mold vertically downwards and is bent only by the subsequent roller guide in the curved mold, the strand is already arc- shaped. Both concepts have their specific advantages and disadvantages resulting from the fact that cracks on the one hand by bending the string and on the other hand a long vertical liquid line provides benefits to the purity of the steel, since impurities can ascend the slag. After complete solidification of the strand is cut on the run-out table with burners in length.

For casting a dummy ( cold leg ) is introduced from the bottom or the top of the mold, which closes the bottom. The filled steel solidifies on the dummy and is drawn off with this down. After reaching the metallurgical length of the dummy and the cast strand can be separated ( uncoupled ).

In continuous casting, we distinguish several processes, which are of the format of the article to be cast strand dependent. In addition to the slab casting for rectangular strands with large width to 2600 mm and less thickness up to 400 mm diameter for sheet metal fabrication, there is also the billets and continuous cast blooms, cast in the round, nearly square, or even profile-shaped cross-sections for the production of rods, wires and profiles be. When slab casting is generally distinguishes the slab casting (thickness about 100 mm to 450 mm), the Vorbrammenguss (thickness from 70 to 300 mm) and thin-slab casting (thickness 3 to 50 mm). In the latter two processes significant savings in the field of systems engineering and the energy balance are possible. A further reduction of metal forming processes is indicated by the thin strip casting ( qv) reached ( thickness < 2 mm).

Vorbandgießen

By Vorbandgießen a slab is created with about 50 mm thickness that can be tapped directly into the finishing train of a hot strip mill. An equalization furnace is only needed between the caster and the roll unit. With this method, a simple inexpensive steel qualities can be processed. Disadvantages are lack of flexibility and surface quality of the tapes.

Thin strip casting

This is not by continuous casting in the traditional sense, since no molds are needed here. It is merely a continuous casting process, which was developed from the continuous strand casting. To separate the name tape casting has been introduced. The method is also not limited to steel, it sat already years earlier for aluminum as a " Caster " by, as it provides better conditions for film production and can operate at a much lower temperature range than that of steel.

In " strip casting " metal strips are produced which can not or only to a few stitches ( passes through a rolling stand ) will be formed.

The now popular 2-roll thin strip casting of steel was patented in the 19th century for Henry Bessemer. He could not, however, be converted into the market. Only since the mid-nineties of the last century it has been used industrially for stainless steels and electrical steels. Here the steel solidifies between two counter-rotating water-cooled rollers and is completely solidified as a band, with max. 1.5 mm thick, is discharged downwards. A certain production problems due to the short residence time of the melt in the gap between the casting rolls. The speed of the rollers must be matched exactly to the casting temperature, as it is not in complete solidification ( casting temperature is too high and / or rolling speed ) breakthrough occurs. If the melt is solidified, however too early, the forces on the rolls too large and can even paralyze the process ( too low casting temperature and / or rolling speed ).

Typically a rolling stand is placed in the system, in which, after a single rolling pass already a strip thickness of 1 mm - at the same time the minimum possible - results. The rolled thin strip is coiled onto a water-cooled rollers, so-called " coils ".

The method has further advantages, the relatively low investment cost, the ability to manufacture smaller lots are cost and the elimination of the re-heating for hot rolling saves time and energy costs.

Secondary metallurgy

Secondary metallurgy, also known as ladle metallurgy is the name given to measures for smelting, serving the quality of being cast steels. Mostly, they are of a physical nature, such as applying a vacuum in order to cause degassing. The " Uddeholm " method sends the same purpose-setting inert gases by means sunken into the pan bottom nozzle through the melt.

Continuous casting of aluminum

Special technique

The continuous casting of aluminum, classified under the top label formats casting can be performed as well as other as a batch or continuous process. In the former case, bars or rods are made ​​of a predetermined length through the system, in the other case the cast strand after a certain length of a flying saw, the technical term is divided and in the continuation of the casting process. The technique may be referred to as semi-continuous, real continuity provides the aforementioned continuous strand.

Regardless of such a system-dependent distinction the liquid metal through a Eingusstiegel or via a runner and a runner system downstream of the mold, or more is supplied to aluminum. The one in the water-cooled mold, said in a primary cooling process externally rapidly solidified strand is withdrawn for further cooling and solidification in a cooling pool (secondary cooling). Reduction and flow in the mold done this in unison. The casting or lowering, which is on average between 5 and 15 cm / min, is determined by the cast alloy.

Different solidification rates of metal melts are always visible in the cast structure. Due to the very fast compared to about sand casting, due to the water cooling solidification there is no phase precipitates, that is, can be found in spectrography of spectral analysis of strand cross sections in the normal case, no differences in the composition. The rapid heat removal during continuous casting, however, prevents a balance of tensions within the solidification structure. Cold aging or heat treatment (solution annealing) eliminate such tensions that may lead to the rolling of cracking. Seigerungsartige exudations in the cast skin are not solved, the cast skin is then milled before further processing. The casting-related errors can be limited, if care is taken to keep the alloy- dependent contraction gap between the solidifying metal and the mold wall is always as small as possible, since air prevents heat dissipation. A much greater problem than the shrinkage gap in the continuous casting of aluminum and its alloys, the sensitivity of the liquid metal with a view to absorption of hydrogen and oxide formation, which require special treatment of the melt in each case.

Melt treatment in continuous casting

The problem of hydrogen uptake, prevention and removal

If the continuous casting is produced in a foundry, which is subordinate to a primary smelter - international fachsprachlich a Cast House - this just before a melt treatment plated metal is delivered, either as pure aluminum, or other already alloyed hut. The treatment takes place in a mixer or holding furnace has the objective to release primarily the melt coming from the electrolysis of contaminating and this means reducing the effects of the content of sodium and calcium. The distance of a given hydrogen content is always makes at the latest possible point. Before the turn of the century, chlorination with chlorine gas or chlorine releasing additives was preferred because it quickly transferred the two impurity elements in their chlorides and let them rise to the slag. Excess aluminum chloride, while adding a useful scavenging effect because it leaves the melt at given temperatures under upsurge. For environmental reasons, the use of chlorine was about 1980 increasingly reduced and argon or freon used with at most lower chlorine addition. The invention of a SNIF box called treatment chamber, which was the distribution system directly upstream, allowing finer metering and a removal of the hydrogen at the last position before casting, so the possibility of a resumption of moist ambient air limiting. Prior art, it is also a required grain refinement to put on a controlled feeding of a wire that is alloyed with fine elements, immediately before the SNIF box. The removal of the oxides is responsible, unless they have been previously rinsed in the course of treatment, filtration ( see below).

Recycling operations, which can also produce continuous cast material from scrap must specifically do in the hydrogen removal a little more because the melted scrap are often corroded, so fraught with a hydroxide, or even oily. On noise disturbances is important also stronger because the recycled material partly comes from foundries, and can contain numerous undesirable elements. Not all have or may be removed. With magnesium or copper contents can provide strangussgeeignetes material, silicon-rich melts are more suitable for the production of Al-Si casting alloys.

The problem of oxide formation, prevention and removal

Given the tendency to oxidation of aluminum melts, especially those with magnesium contents, it is necessary oxidation by application to limit them -inflammatory measures (heat treatment). In the melt, and floating in the cast succeeded oxides, mostly gamma -oxides in the form of cuticle always to casting errors. Even with casting technique is to pay attention to protection against oxidation. You can of continuous cast by decanting and done there in the pouring stream, but also in the distribution system. A good according to state of the art security is provided by filtration.

For the production of aluminum products which are subject to the special quality requirements, be it in formats casting ( semi-finished ) or mold casting, oxide inclusions must be avoided. Extent that oxidation can not be avoided, the melt must be cleaned by rinsing. In the past this has been achieved by a chlorinating treatment of the melt, so the passage of gaseous chlorine, be it from the bottle, or released in situ. However, the chlorination of molten aluminum has several drawbacks in today's perspective. Among other things, the magnesium content of the melt is reduced and, in particular, problems arise in environmental protection, as the educated and volatile at melt temperature of aluminum chloride can be partially released into the environment. Also for workplace hygiene of handling chlorine is undesirable. Therefore today either inert gases or mixtures are used with slight chlorine components. The degassed while purging, the hydrogen content reducing treatment removes the coarser and afflicted with hydrogen oxide. It may be supplemented by an additional filtration, recognized and fine oxides.

Two filtering methods are considered to be state of the art: using either a porous ceramic filter, or a flow-through of the melt filter bed, which consists mostly of petroleum coke. Both methods are suitable trap oxides before entry of the melt into the mold.

A according to its material composition, referred to as ceramic filters for metallurgical use - not to be confused with inserted also in electrical frequency filters of the same name - can both circular, as be square, and has a certain number of pores per cm ² depending on the task on. It is used in continuous casting as close as possible to the distribution system of the continuous casting and allows the flow of a certain quantity of metal, before being replaced (one filter).

In a continuous filtration of this type may still be a counter-current gas of argon or dry nitrogen, can be installed. The content after treatment should be < 0.08 ml/100 g of aluminum.

A beach to tender advantage of filtration is associated with the reduction of the oxide removal resumption of hydrogen. In oxide-containing melts can in fact deposited over the contact surface air melt the existing oxides to form hydroxides and lead to hydrogen contents, which are far above the solubility limit when measuring hydrogen from humid ambient air. As far as technically possible oven, therefore, a protection of the surface of the bath is indicated with suitable mixtures of salts particularly at rest in the holding furnace melting, or alternatively with an inert gas that is heavier than air.