The major areas of research in continuous casting involve ways to decrease the height of the caster and ferrostatic pressure of the caster. This has led to development of horizontal continuous casting to produce some iron and steel shapes. Horizontal casting of steel was first implemented into a plant in the mid 1960s and has since been improved. The first industrial plant to horizontally continuously cast steel parts was a General Motors plant in Lansing, Michigan, in 1971. Since 1975, there have been approximately 30 plants built which horizontally continuously cast iron or steel. However, most of these are only pilot plants. Horizontal continuous casting has several benefits, but also has many drawbacks and problems that have kept it from becoming a more widely used technology.
Shapes And Sizes That Can Be Horizontally Cast
Horizontal casting can be used to produce wire, rods, and billets. The wire can range in size from 3 to 12 mm. One 12 strand wire caster could produce up to 25,000 tons per year. The size range for round or square rods is from 25 mm to 50 mm. Round billets can range from 55 to 330 mm, while square billets can range from 55 to 250 mm.
The wire casters can produce with a higher yield, with less energy, small lots more economically, with a lower capital cost, and with a reduced billet stock when compared to traditional wire drawing methods. Horizontal casting increases the yield by 20 to 35% which is especially important when high alloy steels are being produced. The decrease in energy required is introduced since many of the heating and rolling processing is decreased or eliminated. The continuous caster can easily be turned on and off which greatly increases the ability to make relatively small amounts of a certain sized wire. The billet stock is decreased because the wires are being directly cast instead of drawn from billets.
The first challenge in horizontal casting is taking the vertical stream from the tundish to a horizontal mold. In order to do this, a piece called a break ring is inserted to form a connection between the tundish and mold. The break ring is made of a refractory that is resistant to corrosion and thermal shock and cannot be wetted by the steel. Because of these reasons, the break ring is typically made of either boron nitride or silicon nitride refractory, which possess these capabilities. The break ring must also be machined with high accuracy since it is the connection between the tundish and mold.
Advantages Compared To Traditional Continuous Casting
Horizontal continuous casting has many advantages compared to continuous casting. The most obvious advantage of a horizontal caster is that it can be installed in existing buildings. Traditional continuous casters are usually many stories tall because of the vertical casting portion. The building for a horizontal caster only needs to be tall enough to allow for the movement and positioning of the ladle. Given that there is little to no strand straightening or bending in horizontal continuous casting, high alloy and tool steels can be continuously cast with much greater ease compared to traditional continuous casting. In addition, the ladle and mold are linked by a fixed connection, which eliminates the possibility of reoxidation.
Three advantages are brought about by the absence of any mold oil or flux; the absence of mold oil or flux removes the chance of the formations of mold flux inclusions, there is no carbon pickup from mold flux, and a more intense heat extraction occurs in the mold when compared to traditional continuous casting. There is no need for control of the liquid steel level in the mold since the tundish and mold are united by a fixed connection, which becomes beneficial when casting the smaller cross-sections because the mold level can become very difficult to control for these smaller sections.
Disadvantages And Problems Associated With Horizontal Casting
Many problems are also encountered with horizontal continuous casting as well. The absence of a natural meniscus at the formation of the shell creates a problem. This requires an artificial meniscus, through the use of a break ring, which causes surface cracks or the sticking of the strand shell. This causes a lack of a uniform crystallization pattern which occurs around witness marks. These witness marks are formed by a shell solidifying against the break ring and then being separated by a withdrawal stroke. This lack of a uniform crystallization pattern causes problems with the hot forming properties the high alloy steels.
The absence of mold fluxes has advantages and disadvantages associated with it, the disadvantage being when coupled with high ferrostatic pressures in the skin forming region, high friction is seen between the shell and the mold. To overcome this, the mold materials must possess high abrasion resistance and the mold walls need to be very stable and be made with high dimensional accuracy. The break ring often limits the casting time to a few hours, which becomes an economical problem due to the high cost of the break ring. Horizontal casting also cannot be used on shapes with large cross-sections for the casting of iron and steel.
Horizontal continuous casting also has problems that are seen in traditional horizontal casting. These typical problems are gravity segregation which leads to a grouping of equiaxed crystals in the lower half of the casting and the columnar zone in the upper half. There are also problems with center soundness like what is seen in vertical casting. The mold of the horizontal caster must also be clean of dirt or gas that can become entrapped in the steel; at that point there would be no way to remove the inclusions caused from the dirt or gas from the steel.
There have been some plants that have industrialized the horizontal casting process, the first being a General Motors plant in 1971; however most have remained pilot plants. Horizontal casting can be used to produce wires, rods, and billets. Horizontal continuous casters have some advantages when compared to traditional continuous casters. Some of the advantages are that horizontal casters can be used to cast high alloy or tool steels due to the reduced straightening and bending of the strand, can be installed in pre-existing buildings, and eliminate the possibility of reoxidation of the steel between the ladle and mold. However, there are also many disadvantages associated with horizontal continuous casting of iron or steel. The main disadvantages are associated with the break ring which is used to create an artificial meniscus. The break ring is what typically limits the time allowed to cast. The break ring is also responsible for the formation of witness marks, which causes problems when some of the high alloy steels when being hot formed. Horizontal continuous casting is also limited to smaller section castings when iron or steel is being cast.
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