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.
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Figure 1 Installation location of the intermediate package
1- Ladle; 2- Tundish; 3- Mould; 4- Secondary cooling zone
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
Tundish structure
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.
Figure 2 Schematic diagram of tundish structure
a-Double stream long strip; b-Single stream rectangle
1-pack cover; 2-overflow nozzle; 3-pack wall; 4-pack bottom; 5- nozzle; 6-long nozzle; 7-immersion nozzle; 8-molten steel surface coating
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 °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 ℃ ten (30 -40) ℃. The casting temperature of the tundish is ~°C. To make the temperature reach ~℃, 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.
Figure 3 The law of temperature change of molten steel in the tundish
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.
Figure 4 The relationship between the lining 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 °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~°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.
Figure 5 Composition diagram of refractory materials for tundish
1-cladding; 2-permanent layer; 3-working layer; 4-bag bottom; 5-clad cover; 6-plug; 7-sleeve brick; 8-plug head; 9-seat brick; 10-gate brick
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(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.
The continuous casting tundish is one of the key equipments in the continuous casting process. It has the functions of stabilizing and dividing the flow. It plays a very important role in smoothing the continuous casting operation and ensuring the quality of molten steel. The stable pouring of the tundish is conducive to extending the residence time of the molten steel in the tundish and uniformizing the temperature and composition of the molten steel. However, the working layer near the impact zone of the tundish is eroded by molten steel too quickly, which limits the service life of the tundish. By adding a flow stabilizer in the impact area of the molten steel in the tundish, the erosion of the working layer of the tundish near the impact area by the molten steel can be slowed down, thereby further increasing the age of the tundish. After installing the flow stabilizer, the motion trajectory of the molten steel in the tundish changes, reducing additional circulation, basically eliminating short-circuit flow, reducing slag entrainment, and promoting the floating of inclusions. However, there are still problems such as severe erosion and high temperature at the corners on both sides of the tundish, the seat bricks, and the nozzle, which reduces the service life of the tundish, causes an increase in billet rejection during heat exchange, high consumption of steel materials, and increases in refractory material costs. Therefore, it is necessary to improve and optimize the metallurgical process of the continuous casting tundish to extend the service life of the tundish, ensure the cleanliness of the molten steel, improve the quality of the cast slab, and meet user requirements.
Process flow and existing problems
Process flow: 50t double-blown converter → refining → continuous casting ladle rotary table → tundish → flow stabilizer → water inlet → crystallizer.
1) The working layer of the continuous casting machine tundish has been expanded and transformed many times, and the working lining is relatively thin, with the thickness of the working lining of the tundish being about 80mm. Moreover, the tundish cladding has not been updated after many years of use, and the cladding is severely deformed, resulting in uneven wall thickness. Recently, the corrosion thickness of the slag line in the working layer of the tundish has been more than 40mm after being used for more than 48 hours, and the erosion of the tundish slag line and impact area is serious. After the erosion of the tundish slag line, the temperature measured at this part is more than 50°C higher than that of other parts.
2) At present, in the production of stopper rod tundish of continuous casting machine, the plug head is subject to uneven and strong erosion by the molten steel in the tundish. After 12 hours of use, the stopper rod head erodes unevenly and quickly, causing the stopper rod tundish to be unable to control flow stably, which is a key factor limiting the service life of the stopper rod tundish.
3) At the end of the service life of the sizing tundish, the upper nozzle of the tundish erodes too quickly and the zirconium core falls off. Problems such as nozzle blockage and continuous casting interruption often occur, which seriously affects the efficient production of continuous casting. The above factors lead to frequent unplanned replacement of tundishes, which seriously affects the improvement of the service life of the tundish and brings greater risks to the safety and stable production of the continuous casting machine.
improvement measures
- Tundished slag line magnesia carbon brick composite repair technology
The refractory material of the tundish is divided into two parts: the permanent layer and the working layer. The permanent layer is mainly used for thermal insulation of the tundish and has poor corrosion resistance. The working layer is mainly used to resist the erosion of high-temperature molten steel and steel slag, and the insulation effect is relatively poor. Improving the corrosion resistance of the tundish slag line is mainly improved by optimizing the working layer.
Develop a tundished slag line magnesia carbon brick masonry technology. According to the erosion width of the tundish slag line and the thickness of the tundish working layer, a 300mm×300mm×30mm special magnesia carbon brick for the tundish slag line is designed. The material of the tundish working layer is magnesia dry material. According to the different erosion conditions in the tundish, the material of the magnesia dry material in the slag line of the tundish is improved, and the corrosion resistance of the magnesia dry material in the slag line is improved. Magnesia dry materials are divided into two types: slag line magnesia dry materials and bottom package magnesia dry materials. Magnesium dry materials for the bottom of the tundish are used below the tundish slag line and at the bottom of the tundish, and special magnesium dry materials are used for the tundish slag line. During the preparation of the tundish, after the permanent layer of the tundish is knotted, the working layer of the tundish is knotted. First, fill the bottom of the middle slag line with magnesium dry material. Then place a circle of special magnesia carbon bricks for the slag line along the inner wall of the permanent layer of the tundish, and finally fill the slag line magnesia dry material. After 1.5 to 2.0 hours of baking over medium and low heat, it is formed.
- Combined construction of magnesium plates in the impact area of the tundish
Increase the thickness of the working layer in the impact zone of the tundish. Due to the limited space of the tundish impact zone, the thickness of the permanent layer in the tundish impact zone was reduced from 200 to 220 mm to 150 to 170 mm. The thickness of the working layer in the impact zone of the tundish increases accordingly from 80 to 100mm to 130 to 150mm. Increasing the thickness of the working layer in the tundish impact zone can significantly improve the erosion resistance of the tundish impact zone.
Develop a combined masonry process of magnesium plates in the impact zone of the tundish, and design special magnesium plates for the impact zone of the tundish according to the shape and size of the tundish. The magnesia board is knotted with tundished materials and is divided into three types: outer arc plate, side arc plate and side plate. The thickness of the magnesia outer arc plate and side arc plate is 100mm, and the thickness of the magnesia side plate is 50mm. The outer arc plate, side arc plate and side plate of the magnesium plate in the tundish impact area are combined to protect the working layer in the tundish impact area. After the knotting of the tundish working layer is completed, a flow stabilizer is installed in the impact area of the tundish. The magnesium outer arc plate is installed above the flow stabilizer, and the two magnesia side arc plates are installed on the left and right sides of the flow stabilizer. Two magnesium side plates are installed on either side of the corner of the strike zone. After installation, use a coating material for bonding and bake over medium and low heat for 1.5 to 2.0 hours before taking shape.
- Long-life nozzle for continuous casting tundish
In order to solve the problem of excessive corrosion of the nozzle of the tundish and the shedding of the zirconium core, a new type of long-life nozzle was developed and used. Increase the height of the inverted nozzle, increase the thickness of the magnesium carbon protective layer on the upper nozzle zirconium core, and reduce the erosion of the zirconium core by molten steel; increase the overall thickness of the zirconium core at the nozzle to improve the corrosion resistance of the zirconium core.
1) Design of overall dimensions. The height of the nozzle is increased from 95mm to 145mm. A magnesium carbon protective refractory is added to the upper end of the zirconium core at the upper nozzle. The thickness of the magnesium carbon protective refractory is 20mm, which serves to isolate the zirconium core from the upper nozzle and the molten steel and reduce the direct erosion and erosion of the upper end of the zirconium core by molten steel.
2) Optimization of zirconium material in the inner cavity. The height of the main body of the zirconium core at the upper nozzle is increased from 80mm to 125mm, and the overall thickness of the main body of the zirconium core at the upper nozzle is increased by 2mm. The inner hole at the upper end of the zirconium core at the upper nozzle that is in direct contact with the molten steel is designed as an R arc with a radius of 5mm to improve the strength of the zirconium core at the upper nozzle. And buffer the impact of molten steel on the zirconium core.
Stopper rod tundish combined with cofferdam seat brick
The quality of the stopper rods, seat bricks, and nozzle refractory materials used in the continuous casting tundish determines the service life of the stopper rod tundish. The seat brick is used in conjunction with the nozzle of the tundish. Its main function is to fix and protect the nozzle of the tundish, and improve the installation accuracy and service life of the nozzle. The stopper rod cooperates with the nozzle of the tundish to control the flow.
In order to solve the problem of uneven and rapid erosion of the stopper rod head of the stopper rod tundish, a combined cofferdam seat brick with stopper rod tundish was developed. The modular seat brick is divided into two parts. The lower seat brick is used to position the tundish seat brick and the nozzle. The upper seat brick is higher than the matching nozzle bowl of the tundish, and plays a role in protecting the nozzle bowl of the tundish and the stopper rod head. The combination of the nozzle of the stopper rod tundish, the stopper rod, and the combined cofferdam seat brick solves the problem of uneven and rapid erosion of the stopper rod head by molten steel in the tundish, and increases the service life of the stopper rod tundish.
Implementation Effect
1.The composite masonry process of the tundish working layer greatly improves the erosion resistance of the tundish slag line. After the implementation of the process, it was observed that the tundish was used for 48 hours. The erosion of the working layer at the slag line of the tundish disappeared and the shape of the magnesia carbon bricks was intact. It was observed that the tundish was used for 70 hours. Some magnesia carbon bricks remained at the slag line of the tundish. After the tundish was turned over, , check that the permanent layer is intact.
2.The combined masonry process of magnesium plates in the impact zone of the tundish improves the erosion resistance of the impact zone of the tundish.
After the process was implemented, it was observed that the tundish was used for 48 hours. The shape of the magnesium carbon plate in the impact area of the tundish was obviously eroded, and the working layer was intact. It was observed that the tundish was used for 70 hours. The magnesium carbon plate in the impact area of the tundish disappeared and the erosion thickness of the working layer was 100~150mm. , in , the number of tundishes that were offline due to severe erosion in the impact zone was reduced to zero.
3.The application of the stopper rod tundish combined cofferdam seat brick effectively solves the problem of uneven and rapid erosion of the stopper rod head in the stopper rod tundish, and greatly improves the corrosion resistance of the stopper rod head. After the process was implemented, it was observed that the stopper rod tundish was used for 11 hours, and the rod head of the plug rod tundish was slightly eroded; it was observed that the plug rod tundish was used for 18 hours, and the rod head of the plug rod tundish was significantly eroded, but it was obviously lighter than before optimization and can continue to be used.
4.The application of the new long-life nozzle reduces the erosion of the zirconium core by molten steel. After 72 hours of on-site use, the nozzle of the tundish did not burst, and the entry of foreign matter into the nozzle was reduced to once per month.
By applying the magnesia-carbon brick composite repair technology of the tundish slag line, the magnesia plate combined repair technology of the tundish impact area, the long-life nozzle technology of the continuous casting tundish, and the plug-rod tundish combined cofferdam seat brick technology, Laiwu Branch The overall erosion resistance of the tundish in the steelmaking plant is improved, the refractory material in the impact zone is stable, the cleanliness of the molten steel is ensured, the metal yield is increased, and the average life of the sizing tundish is increased from 48h/package to 72h/package.
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