Today, a huge number of a variety of things, parts, etc. are made of steel. Naturally, this requires a large amount of source material. Therefore, the factories have long been using the method of continuous casting of steel, characterized by the most important sign - high productivity.
Basic equipment for work
To date, it is known about several installations for casting steel in this way, and in short they are called UNRS. Initially, a vertical type installation was developed and put into production, located below the workshop floor level by 20-30 meters. However, then the main engine for the development of these installations was the desire to abandon the deepening of the floor. This led to the development and implementation of installations for the continuous casting of tower-type steel. The height of these installations was 40 m. However, this embodiment of the machine was not widely spread for two reasons. Firstly, to build such an assembly in the workshop is quite problematic and time-consuming. Secondly, even more difficulties arose with its operation.
Bending and radial installations
Over time, the continuous casting of steel was transferred to work with vertical bending machines. The main feature is a 90 degree bend of the ingot leaving the shafts. After that, the installation used the special correct mechanism for straightening the ingot, and only after this stage the cutting took place. Continuous steel casting on such equipment did not become too popular for some reason. Firstly, the bend, of course, allowed to reduce the height, but at the same time greatly limited the cross section of the ingot itself. The more it was necessary to obtain a cross-section of the material, the more there was to be a bend, which means that the height increased again. Secondly, machines with a bend were placed in steelmaking workshops with even greater difficulties than vertical ones.
Today, installations for continuous casting of radial steel are becoming increasingly popular. On such an aggregate, an ingot is formed in the mold and leaves it along the same arc along which it fell into it. After that, he will straighten with a pulling-correct mechanism. And then you can start cutting the ingot into billets. It was such a design in practice that turned out to be the most rational when organizing freight traffic in the steelmaking shop.
How does casting begin?
The technology of continuous casting of steel is a rather complicated process. However, it will be fair to say that the principle remains the same despite the installation used for production. You can consider the technology on the example of a vertical UNRS.
A ladle for casting steel is fed to the machine through a special crane. After that, the steel flows into the intermediate bucket having a stopper. For equipment with one stream there will be one stopper, for multi-grooved machines - one stopper for each stream. In addition, the tundish has a special partition to hold the slag. From the intermediate ladle, steel will flow into the mold, passing for this through a metering cup or locking device. It is important to note here that before the first casting, seed is introduced into the mold from the bottom side. It fills either the cross-sectional bar of the entire mold, or only the shape of the workpiece. The top layer of the seed will be the bottom of the mold. In addition, it also has the shape of a swallow tail for future coupling with the ingot.
Further casting
Further, in the process of continuous casting, it became necessary to wait until the level of raw materials rises above the seed to a height of about 300-400 mm. When this happens, the mechanism that drives the pulling device starts. It has pulling rolls, under the influence of which the seed will lower and pull the created ingot along with it.
In a continuous casting machine, a mold is usually made of copper with hollow walls. It is under the intense influence of cooling water, and its internal section corresponds to the shape of the ingot, which must be obtained. It is here that the ingot crust is formed. At high casting speeds, this crust may break and metal leak. To avoid this, the mold is characterized by reciprocating movements.
Mold performance
In the installation of continuous casting of steel there is an electric motor responsible for creating such a reciprocating motion. This is done through the force of a gear with a cam-type rocking mechanism. First, the mold moves in the same direction as the workpiece, that is, down, and after the process is completed, it returns back up. Swing stroke is from 10 to 40 mm. The mold is an important compartment in the continuous casting of steel on any type of equipment, and therefore its walls are lubricated with paraffin or any other lubricant suitable for the characteristics.
It is worth noting that in modern equipment the metal level is controlled radiometrically, applying a control signal to the ladle stopper. In the crystallizer itself, either a neutral or a reducing atmosphere can be created above the metal level to avoid oxidation of the product during its production.
Ingot crust
It is worth noting that work under vacuum is also considered a promising method of casting. One plant can cast through several molds at once. Thus, the number of streams of one installation can reach up to eight.
To form the lower part of the peel of the ingot, the action of heat removal of the cold seed is used. The ingot will exit the mold under the influence of a seed, which is drawn into the secondary cooling zone (ZVO). In the middle of the workpiece, the steel will still be in a liquid state. It is important to note here that, at the request of steel casting technology, the thickness of the crust should be at least 25 mm at the time of exit from the mold. To satisfy these requirements, it is necessary to choose the right speed of the material.
Characteristics of the installation and the casting process
Technological characteristics are approximately as follows. If the cross-section of the ingot is 160x900 mm, then its speed should be from 0.6 to 0.9 m / min. If the cross section is 180x1000 mm, then the speed decreases to 0.55-0.85 m / min. The highest speed indicator is required when the cross section of a square type ingot is 200x200 mm - 0.8-1.2 m / min.
Based on the above indicators, it can be concluded that the average casting speed of one stream using continuous casting technology is 44.2 t / h. If you exceed the optimal rate, then the central porosity will increase.
Further, it is worth noting that the temperature of the metal affects the stability of casting and the quality of the product itself. It was experimentally established that at a temperature of more than 1560 degrees Celsius the surface of the ingot is often covered with cracks. If the temperature is lower than indicated, then often happens tightening the glass. Thus, it was found that the optimal temperature for the method of continuous casting of steel will be 1540-1560 degrees Celsius. To maintain this indicator, the heating temperature of the furnace before exhaustion should be in the range of 1630-1650 degrees.
Secondary cooling zone
In this area, the most intensive and direct cooling of the ingot is carried out using water coming from the spray. There is a special system of single, not power rollers. Their rotation prevents the bending of the ingot or its warping. Due to the intense cooling in this zone, the walls of the ingot will quickly add to the thickness, and crystallization will spread deep. The speed of drawing the ingot and the degree of its cooling should be selected so that by the time the ingot arrives on the pulling rolls, it is already completely solid.
What are the benefits of continuous casting?
Since this method of casting steel has replaced the method of casting into molds, it is worth comparing with this method. In general, it is worth highlighting such advantages: greater productivity, lower costs and lower labor intensity of the process. Due to the continuous formation of the ingot, the shrink shell is transferred to the tail section, in contrast to the molds, where each ingot had its own shell. Because of this, the percentage of yield of metal significantly increases. UNRS allows you to get a workpiece of a very different shape, from a small square of 40x40 mm to a rectangle of 250x1000 mm. The use of machines for continuous casting made it possible to completely abandon crimping mills. This significantly reduced the cost of the production process, and hence the price on the market. In addition, the process of metallurgical redistribution was simplified.
disadvantages
Despite the possibility of high mechanization and automation of the process, the high percentage of suitable ingots and other advantages described above, this method has some negative aspects. The disadvantages of continuous casting of steel are as follows.
Firstly, it is not possible to produce ingots of complex configuration. Secondly, the nomenclature of ingots and billets is quite limited. It is quite difficult to convert machines for casting raw materials of another brand, which can increase the final cost of a product of a different brand if it is produced at the same factory. Some steel grades, for example, are boiling, and it is completely impossible to manufacture using this method.
The last disadvantage of the continuous method of casting steel is very significant. It is a possible breakdown of equipment. Failure of UNRS will lead to huge losses in productivity. The longer repairs take place, the stronger the losses will increase.