Ways stell casting
Steel casting is a complex, critical process that directly affects the quality of the metal. It directly affects the following steel characteristics: the number of foreign inclusions, chemical heterogeneity, gas saturation, structure, and surface quality of the billets. The fact is that during casting, various physical and chemical reactions occur in the metal. They, in fact, determine its final quality.
Casting can be carried out by one of three existing methods:
– So-called “raining,” or pouring into molds from above;
– “Siphonic” or from below into bottomless molds;
– Continuous casting in water-cooled molds.
The continuous casting method (the latter), due to a number of advantages over pot casting, is used more frequently in steelmaking.
The “rain” casting method
This method uses conventional carbon steels. The molten metal in rain pouring is fed directly from the ladle into the mold. The advantages of this method include easy preparation of equipment for the process, lower steel temperature than the bottom pouring method, and elimination of material consumption for sprues.
The method also has one, but significant disadvantage. During casting, the steel is poured into the mold from a great height. This creates a splash film which hits the mold walls and hardens quickly. These spatters make the subsequent machining of the castings very difficult, deteriorating their surface due to the formation of oxide films on it. Even rolling does not drain these films from the “body” of the casting, so special surface cleaning is required, which is a very time-consuming operation.
Casting from below
Alloy and high-alloy steels are used for casting from below. Siphon casting involves filling several molds with liquid steel simultaneously. The number ranges from four to sixty. This method is based on the principle of communicating vessels. All the molds are positioned on a special pallet with a central riser, where the molten metal flows in. The molds are filled through channels from below. This ensures smooth, splash-free filling of the molds and gives the castings a clean surface.
In addition, a clear advantage of this method is a significant reduction in casting time, since several small mold cavities can be filled simultaneously with a large quantity of metal.
However, it also has its disadvantages. The siphon feeding of steel increases the labor intensity of the preparation process, increases the consumption of refractories and metal for sprues, and necessitates heating the metal to higher temperatures, since the steel cools quickly while flowing through the channels.
This is a more advanced method. The process looks like this: molten steel flows continuously into the water-cooled crystallizer, which has no bottom. The ingot, with its solidified periphery and liquid core, is pulled out of the lower opening of the mold. It then passes through the secondary cooling zone, where it solidifies completely and is fed for cutting with a gas cutter that divides it into blanks of the desired length.
The process continues until the pouring ladle is empty. A dovetail-shaped seed is introduced into the mold before casting begins. It is pulled out of the mold together with the casting.
This method produces ingots with a dense, fine-grained structure, without shrinkage shells, and with a high-quality surface. These properties are given to the castings through a directed solidification process and continuous feeding during shrinkage. There are practically no rejects at the output. The number of good products reaches 98% of the weight of the cast metal.
The continuous casting machines are equipped with two to eight casting molds. This makes it possible to make several ingots simultaneously, which also has a positive effect on the speed of obtaining high-quality results.