Equipment details of CCM

 Some of these equipments are described in more detail below.

Ladle turret

One very important part of a CC machine is the ladle turret. It is mounted at the reinforced concrete base. It holds the steel teeming ladles, which can weigh up to 300 t. By means of the ladle turret, the steel teeming ladles are alternately slewed into pouring and charging position. This function ensures the uninterrupted operation of the CC machine. While one ladle is being emptied, a full ladle is provided on the other side.

The bearings in the ladle turret, in spite of being subjected to high forces and considerable tilting moments, reach service lives of more than 10 years.

Ladle turret supports the ladles and its hydraulic system with rotary arms has the mechanism to allow the ladles to be raised and lowered whilst maintaining a horizontal position. Also strain gauge load cell is incorporated in the ladle turret to allow the weight of the ladles to be continuously monitored. Variable frequency AC motor is normally used for the transmission mechanism. Ladle turret usually has emergency response mechanism available for ensuring the safety of operators in an emergency. It also generally has manhole which ensures its easy maintenance. It is also normally equipped with the ladle cover manipulator.

Tundish

The main functions of the tundish are to be a steel reservoir between the steel teeming ladle and the mould, and in the case of multi strand CC machines to distribute the liquid steel into the different moulds. The first item is of special importance during the ladle change. In addition to being a reservoir of liquid steel, the tundish is more increasingly being used as a metallurgical reactor vessel aimed at improving control of steel cleanliness, temperature, and composition.

Tundishes are usually of an elongated, geometrically simple shape. There are many types and shapes of tundish. One common tundish design for multi strand billet and bloom CC machines is a trough shape with a pouring box offset at the midpoint while for the slab CC machines the tundish is a short box or of a tub shape. The pouring stream from the ladle is directed downward to a position in the tundish bottom which is protected with a wear resistant pouring pad. This position is usually as far as possible from the tundish nozzle to minimize turbulence. In other locations, the tundish is lined with refractory bricks or boards. Weirs and dams are used as flow control devices which both increase the residence time as well as reduce the detrimental effects of turbulence on the liquid steel surface, the liquid steel streams entering the mould and dead zones.

Nozzles for protecting the pouring stream against reoxidation between ladle and tundish and tundish and mould are used nowadays almost on all the CC machines, at least when casting high grade steels. Both stopper controlled nozzles and slide gates of various designs are used to control the steel flow from the ladle to the tundish and from the tundish to the mould. The free surface of the liquid steel in tundish is generally covered with slag to avoid reoxidation and heat losses from the liquid steel.

The discharge rate of liquid steel is controlled by the bore of the nozzle and the ferrostatic pressure (height of liquid steel in the tundish) above the nozzle. Different bores are selected depending on the section size being cast and casting speed required. Stopper rod controlled nozzles are used for casting slabs and large sections when aluminum killed steels are produced. In this application, discharge rate of liquid steel through the nozzle is controlled manually or automatically by the setting of the stopper head in relation to the nozzle opening. Earlier oversized nozzles were used for casting aluminum killed steels because of the buildup of alumina so that the stopper head could be raised to compensate for a reduction in flow rate.

Recent developments in deoxidation practices together with the use of argon bubbling through the stopper head and nozzle units have minimized the alumina buildup problem. Another development in controlling liquid steel flow from the tundish is the application of slide gate systems which are similar to those employed on ladles. These gate systems can also provide the capability for changing nozzles during casting as well as changing nozzle size.

Tundish car usually adopts the half suspended design and is mounted at the main operating platform. It is usually hydraulic powered and is used to support and convey the tundish for casting or heating. It also incorporates weighing mechanism for the weight measurement to allow the weight of liquid steel to be continuously monitored.

Mould

The mould is the heart of the CC machine and the origin of many defects can be related to the phenomena taking place in the mould. Hence the mould phenomena and control of them are of special importance. The main function of the mould is to establish a solid shell sufficient in strength to contain its liquid core upon entry into the secondary spray cooling zone. Key product elements are shape, shell thickness, uniform shell temperature distribution, defect free internal and surface quality with minimal porosity, and few non-metallic inclusions.

The mould is an open ended box structure which contains an inner lining fabricated from a copper alloy which serves as the interface with the liquid steel being cast and provides the desired shape to the cast section. The liner is rigidly connected to an outer steel supporting structure.

Moulds can be tubular moulds or plate moulds, and depending on the type of the CC machine, they can be straight or curved. For larger strand cross sections, as for slabs, plate moulds are normally used. The mould material has to meet many requirements. Mould materials usually consist of copper and some copper alloys. To avoid wearing of the copper material, the moulds are typically coated with chromium or other hard material. The mould is cooled by water and this cooling is called primary cooling. To avoid boiling or bubble formation in the water channels, which makes the cooling unstable, the water velocity in the channels is to be fast enough, even up to 10 m/sec or more and the water temperature must not exceed 50 deg C. It is also important that the water is clean and any deposit cannot be accepted on the cooled surface.

The steel shrinks as it solidifies and cools. As a result, the moulds are normally tapered or multi tapered to compensate for the strand shrinkage as well as to ensure a good contact between the mould and the shell and so to ensure a good and smooth heat transfer from the shell to the mould. To prevent the high friction between the mould and the steel, the mould is oscillating and the casting powder (or oil in some cases) is used as a lubricant. Casting powder is very effective to keep mould friction low and strand surface quality high. Casting powder is added on the steel surface manually or using automatic powder feeders. It is important to have a stable pool of liquid casting powder on the top of the steel level to ensure the constant and smooth feeding of the liquid powder into the mould-steel interface.

There are two types of mould design namely (i) tubular mould, and (ii) plate mould. Tubular moulds conventionally consist of a one piece copper lining that usually has relatively thin walls and is restricted to smaller billet and bloom casters. Plate moulds consist of a 4 piece copper lining attached to steel plates. In some plate mould designs, opposite pair of plates can be adjusted in position to provide different section sizes. For example, slab width can be changed by positioning the narrow face plates, and the slab thickness can be changed by altering the size of the narrow face plates. The plate mould is inherently more adaptable than the fixed configuration, tubular mould. In addition to permitting size changes, changes can also be made to the mould taper (to compensate for different shrinkage characteristics of different steel grades) as well as ease of fabrication and reconditioning.

During the casting operation, the copper liner is subjected to distortion (a change in the internal dimensions of the mould). It is caused mainly by mould wear and mold deformation due to thermal and mechanical strains.

Control of heat transfer in the mould is accomplished by a forced convection cooling water system, which is normally designed to accommodate the high heat transfer rates that result from the solidification process. In general, the cooling water enters at the mould bottom, passes vertically through a series of parallel water channels located between the outer mould wall and a steel containment jacket, and exits at the top of the mould. The primary control parameters are namely (i) the volume of water at the required water temperature, pressure and quality, and (ii) the flow velocity of water uniformly through the passages around the perimeter of the mould liner.

Mould oscillation is necessary to minimize friction and sticking of the solidifying shell, and avoid shell tearing, and liquid steel breakouts, which can wreak havoc on equipment and machine downtime due to clean up and repairs. Friction between the shell and mould is reduced through the use of mould lubricants such as oils or powdered fluxes. Oscillation is achieved either hydraulically or via motor-driven cams or levers which support and reciprocate (or oscillate) the mould.

Motor driven cams, which support and reciprocate the mould, are used primarily. Mould oscillating cycles are many and varied with respect to frequency, amplitude and pattern. Many oscillation systems are designed so that the cycle can be changed when different section sizes on steel grades are cast on the same CC machine. However, there is one feature that has been adopted, almost without exception, which applies a negative strip to the solidifying shell. Negative strip is obtained by designing the ‘down stroke’ of the cycle such that the mould moves faster than the withdrawal speed of the section being cast. Under these conditions, compressive stresses are developed in the solidifying shell which tends to seal surface fissures and porosity and thus enhance the strength of the shell. During the ‘up stroke’ portion of the cycle, the mould is very rapidly returned to the starting position and the cycle is then repeated. Thus the shape of the oscillating cycle is non?symmetrical with respect to time.

Electromagnetic stirring (EMS) systems create the electromagnetic force, which works on every unit of volume of steel and bring about a stirring motion in the liquid steel. An EMS system consists of (i) power pack including transformer and high and low voltage power distributor, (ii) frequency converter,(iii) stirrer, (iv) monitor/controller, and (v) cooling water system. The application of electromagnetic stirring (EMS) technique promotes the formation of an equi-axed crystallic zone in the strand. It causes the refinement of the solidification structure, the reduction in the content of inclusions and improvement in the quality of the surface, sub surface and the inner structure of the cast product.

Secondary cooling

Typically, the secondary cooling system is comprised of a series of zones, each responsible for a segment of controlled cooling of the solidifying strand as it progresses through the machine. The sprayed medium is either water or a combination of air and water.