The tundish is a transition device for molten steel between the ladle and the mold. After the tundish bears the molten steel flowing in from the continuous casting ladle, it plays the role of “connecting up and down”. The installation position is shown in Figure 1.
The main tasks of the tundish are: (1) Diversion of molten steel. For the multi-stream continuous casting machine, the molten steel is distributed to each mold through the tundish; (2) steady flow. Reduce the static pressure of the molten steel, maintain a stable molten steel level in the tundish, and smoothly inject molten steel into the mold; (3) store molten steel. When replacing the ladle with multiple furnace continuous pouring, the drawing speed is not reduced, creating conditions for multiple furnace continuous pouring; (4) Purifying molten steel. In a long pouring time, the temperature of the molten steel is basically unchanged, and the inclusions in the molten steel are further floated, preventing the molten steel from contacting the air, and avoiding oxygen and nitrogen absorption
The tundish is generally composed of a package body, a cover, a nozzle and a stopper rod, etc. The outer shell of the package body is a metal structure, and the inner lining is a refractory material. It is generally rectangular, but also has a “T” shape, a trapezoid shape or a “V” shape. Its purpose is mainly to reduce the eddy current generated when molten steel is injected, and also to clean the slag of the ladle, and it is convenient to operate when hanging.
The shell of the tundish is made of welded steel plates. It requires sufficient rigidity so that it does not deform during high-temperature work, handling, and cleaning. For this reason, reinforcement ribs are welded on the outside of the shell. Blocks are provided in the large-capacity tundish. The flooded wall is used to isolate the disturbance of the molten steel in the tundish by the steel flow from the ladle. In order to reduce the heat dissipation of molten steel, the tundish should be covered with heat preservation.
The structural parameters of the tundish are mainly the length, width and capacity of the tundish. The length of the tundish mainly depends on the flow number and flow spacing of the casting slab. The nozzle distance from the end of the package wall is generally not less than 200mm. With these two sizes, it can be To determine the length and width of the tundish, the main consideration is that the distance between the molten steel injection position and the nozzle should be beneficial to the distribution of molten steel, and the molten steel does not form a dead angle in the tundish, and the distance from the impact point of the molten steel to the center of the nearest nozzle should not be less than 500mm. Excessive body width will increase the heat dissipation area and reduce the thermal insulation performance of the tundish. At the same time, it will increase the weight of the tundish body, increase the gauge of the tundish car, and affect the layout of the tundish supporting equipment. The tundish capacity is generally 20% to 40% of the ladle. In recent years, there has been an increasing trend. When multiple furnaces are continuously poured, the molten steel stored in the tundish should be able to ensure normal pouring for 5 minutes.
According to the role of the tundish, its structure should meet the following requirements: Strive to have a small heat dissipation area, good thermal insulation performance, simple appearance, convenient for bricklaying, ladle cleaning and pouring operations, and the layout of the nozzle should meet the requirements of the cast slab section and flow number. Under long-term high temperature, the structure is stable and reliable.
The commonly used tundish shape and size are determined by the position of the outflowing molten steel stream and the number of streams. Multi-strand continuous casters usually use long strip tundishes, and rectangular tundishes are only suitable for single-strand continuous casters. The size of the tundish capacity is determined by the continuous casting speed. Generally, it should be slightly larger so that the casting can be continued when the ladle is replaced. At the same time, there must be sufficient static pressure head to ensure stable outflow of molten steel, reduce turbulence and facilitate the floating of non-metallic inclusions. Therefore, the tundish wall and the bottom are inclined, and sometimes a slag retaining wall or partition wall is also set. The schematic diagram of the tundish structure is shown in Figure 2.
The inner lining of the tundish is made of refractory bricks, and the inner wall has a certain taper to facilitate slag removal and brick surveying to squeeze the asbestos board between the shell and the lining to reduce heat dissipation. The bottom of the ladle is equipped with one or more nozzles, and the top of the ladle is provided with a cover. The purpose is to protect the bottom of the ladle during heat preservation and casting, so that it will not be deformed due to overheating and baking. Clean the tundish before pouring, and bake the inner lining to about 1100°C to prevent the nozzle from freezing when pouring is started.
The technical operation of the tundish is essential to ensure the smooth operation of continuous casting. The following points must be paid attention to during operation:
(1) The tundish nozzle is smaller than the ladle nozzle and is easy to be blocked by cooling.
(2) The height of molten steel in the tundish is used to adjust and control the flow of molten steel in the tundish into the mold. The high-height molten steel has a large outflow, and the shallow molten steel has a small outflow.
(3) The height of the molten steel liquid level is generally 550mm, not less than 300mm.
(4) The pouring temperature (temperature in the tundish) varies with different steel grades. The most suitable temperature is: the freezing point of steel + (30 -40) ℃. For example, the freezing point of a certain steel grade is 1510 ℃ ten (30 -40) ℃. The casting temperature of the tundish is 1540~1550°C. To make the temperature reach 1540~1550℃, the height of molten steel should be adjusted according to the situation.
(5) The temperature difference of molten steel in the tundish is about 5~10 ℃, such as a 3-strand continuous casting machine; the temperature of the second-stream is higher, and it is easy to leak; the lower temperature of the first-stream and the third-stream are easy to block the nozzle.
(6) The law of temperature change of molten steel in the tundish is shown in Figure 3.
The temperature change of the molten steel in the ladle along the height of the ladle wall determines the change law of the temperature of the molten steel in the tundish during the continuous casting process. Generally 15-20 minutes after pouring starts, the temperature of the tundish lining gradually rises, and the temperature of the molten steel is also higher than the previous period. In the middle of pouring, as long as the tundish steel level is maintained at a stable height, the temperature of the tundish steel level will generally not occur. Big fluctuations. At the later stage of ladle pouring, the temperature of the molten steel in the tundish decreases accordingly. However, since the temperature of the inner lining of the tundish has increased, the temperature drop is less than in the previous period.
The preheating degree of the tundish before continuous casting is the main factor that affects the temperature drop of the molten steel in the early stage of the tundish. When the temperature of the working surface of the tundish lining increases by 300~500 ℃, the temperature loss of the molten steel in contact with the tundish lining can be reduced by 20%~ 25%. Figure 4 shows the relationship between the preheating temperature of the tundish and the temperature of molten steel.
Tundish type and lining structure
According to the cast steel smelting method and whether it needs to be baked and other conditions, the tundish is roughly divided into the following types:
(A) The high-temperature tundish is set up for a specific metallurgical process with a magnesia brick lining top heated to about 1500°C.
(B) Hot tundish, which is a common tundish, uses fired bricks or unfired bricks or castables as the lining, and is preheated to 800~1100°C before pouring.
(C) The cold tundish is lined with insulation board, which can be used without preheating before pouring. The tundish is lined with refractory materials and generally includes the following parts, as shown in Figure 5.
(1) Insulation layer (10~30mm), this layer is next to the steel shell of the tundish, usually with asbestos board, insulation brick or light castable. The best effect is the thermal insulation fiberboard, which is about 12mm thick, has low thermal conductivity, and is easy to build.
(2) Permanent layer (100~200mm), this layer is in contact with the thermal insulation layer. The material is generally clay bricks. The overall permanent lining is the most common. The castable is generally high aluminum.
(3) Working layer (20-50mm), this layer is in contact with molten steel and is the key part. The lining materials include semi-silica, waxstone, clay bricks, high alumina bricks, alkaline bricks (such as magnesia bricks and other zircon bricks. Or use heat insulation boards: siliceous heat insulation boards, magnesia heat insulation boards, magnesium Olivine insulation board. Or paint: magnesia, magnesia chromium and magnesia calcium paint, etc. At present, castables are also used as tundish linings.
(4) The seat bricks are inlaid at the bottom of the tundish and used to install the nozzle of the tundish. The material is usually high aluminum.
(5) The material of the bottom of the package is basically the same as that of the working layer. The working layer of the bottom of the tundish is most easily damaged by the impact of molten steel and requires corrosion resistance and abrasion resistance. Generally, high alumina bricks or dense high alumina bricks and aluminum chrome bricks are used. Special large high-strength tar magnesia bricks are also used. Zircon bricks are also used to strengthen the impact area of molten steel.
(6) The cover is covered on the tundish, which can play the role of heat preservation and prevent the dripping of molten steel. The material is usually clay or high-alumina refractory castable as the cover.
(7) Slag retaining wall (weir). The wall is built in the tundish. It can be a single wall or a double wall. The material of the slag retaining wall (weir) is usually high-alumina bricks, or it can be made into prefabricated blocks. , Its purpose is to block slag. In order to improve the cleanliness of molten steel, a molten steel filter can also be installed on the slag retaining wall.
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