Causes and control of liquid level fluctuations in round billet continuous casting mold

Abstract: Combined with the actual situation of the round billet production of Maanshan Iron and Steel Co., Ltd., the factors affecting the fluctuation of the mold liquid level in round billet continuous casting were systematically analyzed, and corresponding control measures were taken to effectively control the mold liquid level fluctuation. The quality stability of the billet has been greatly improved, the rate of downgrading and correction of the billet and the scrap rate have been greatly reduced, which provides a guarantee for the stable pouring of the round billet.

Keywords: round billet; copper mould tube; liquid level fluctuation; control measures

During the continuous casting production process, the copper mould tube liquid level fluctuates, which not only hinders the smooth flow of production, but also affects the quality of the slab to a large extent. As the copper mould tube liquid level fluctuation intensifies, the content of non-metallic oxide inclusions existing under the cast slab will significantly increase, thereby deteriorating the surface quality of the finished cast slab; at the same time, the fluctuation of the mold liquid level will also cause the mold to The slag entrainment of the molten steel in the cast slab causes the oxide inclusion content inside the slab to exceed the standard. In severe cases, vicious production accidents such as longitudinal cracking, steel leakage or slag leakage may occur. Therefore, in order to improve continuous casting production efficiency and obtain good slab quality to meet customer needs, it is necessary to study the causes of mold liquid level fluctuations and formulate corresponding control measures.

Maanshan Iron and Steel Special Steel Co., Ltd. uses a five-machine five-strand continuous casting machine designed by Danieli in Italy. It mainly produces steel for rail transit, steel for automobiles, steel for energy, and steel for high-end manufacturing. It has very high requirements for the quality of the cast slabs. Strictly, the liquid level fluctuation exceeds a certain range, which directly affects the quality of the slab. Based on the actual production of Maanshan Iron and Steel Co., Ltd., the author studies the influencing factors of liquid level fluctuations in the round billet continuous casting mold and proposes control measures to improve liquid level fluctuations.

Round billet continuous casting production process

The process flow of continuous casting round billet of Maanshan Iron and Steel Co., Ltd. is: initial refining in electric arc furnace – LF refining and composition – RH degassing treatment – CCM five-machine five-strand round billet continuous casting machine – slow cooling of the cast billet – storage. Its main parameters are as follows:

Annual output: 800,000 tons;

Steel ladle capacity: 115 t;

Casting machine flow number: 5 flows;

Cast slab cross-section diameter: 380 mm, 450 mm, 500 mm, 600 mm, 700 mm;

Fixed length of billet: 3700~10000 mm;

Pulling speed range: 0.18~0.6 m/min;

Casting cycle: 45 minutes;

The average number of designed continuous pouring furnaces: 10 furnaces.

Fluctuation of liquid level in round billet continuous casting

During normal production of the casting machine, a target mold liquid level value will be set. When the actual liquid level of molten steel in the mold is more than 3 mm lower than or higher than this set value, and changes repeatedly, with fluctuations lasting longer than 20 seconds. , severe or even lasting conditions for the entire pour are defined as copper mould tube liquid level fluctuations. According to the actual on-site production of Maanshan Iron and Steel Company, the characteristics of liquid level fluctuations in the round billet mold are summarized as follows:

(1) Occasional, it does not occur every pouring time, and the time period of occurrence is also accidental. Sometimes it does not occur for a period of time, and sometimes it occurs continuously.

(2) The first furnace will have larger and more frequent liquid level fluctuations.

(3) In some cases, the liquid level fluctuates first, and then the rod position fluctuates to maintain the stability of the liquid level; in others, the rod position fluctuates first, and then the liquid level fluctuates.

(4) Liquid level fluctuations have a certain relationship with steel type.

Causes of liquid level fluctuations in round billet continuous casting

The mold liquid level fluctuates due to over-tensioning of the dummy bar by the leveler

During the production process, intensive and frequent liquid level fluctuations will occur during the first furnace operation, often accompanied by large liquid level fluctuations exceeding ±3 mm, with strong regularity.

Continuously track 253 single-fire heats, enlarge statistics on such liquid level fluctuation curves, and calculate the distance traveled by the slab from the start to the time when the liquid level fluctuates, as shown in Table 1.

Based on the above statistics of single-heat furnace runs, it is found that the first large liquid level fluctuation will occur when the slab travels to 16.5 m. This length is exactly where the connection between the dummy transition section and the dummy rod body reaches the position of the #1 tension leveler. Due to deformation of the dummy rod joint or scrap steel, residue, etc. stuck on it. The dummy rod generates a certain resistance at the pit of the transition section, causing the actual liquid level to change.

Nodules and nozzle blockage cause copper mould tube liquid level fluctuations

The steel produced by Maanshan Iron and Steel Co., Ltd. basically uses aluminum deoxidation. The Als content in the steel is relatively high, and some of it is S-containing steel. According to some customer requirements, RH non-calcium treatment process is adopted. This type of steel has poor fluidity. At the same time, poor protective pouring results in secondary oxidation of molten steel and worsens the fluidity of molten steel. Non-calcium-treated high-aluminum steel and sulfur-containing steel will react (1) and (2). If the protective casting is not done well, reaction (1) will occur:

4Al+3O2=2A12O3 (1)

Ca+S=CaS (2)

The high melting point inclusions A12O3 and CaS produced by the reaction continue to adhere to the plug head, the nozzle bowl and the nozzle inner wall as the casting progresses, causing the nozzle to be clogged. The pulling speed does not match the steel throughput. In order to keep the liquid level within the set value range, the stopper rod automatically lifts up. Nodulation and clogged water inlets are a slow process. However, the process of peeling off and unblocking the nodules is very rapid. When the nodules are washed away or the nozzle blockage is released, the stopper rod position will suddenly drop, manifesting as a sudden and severe liquid level fluctuation. During the production process, nodules are constantly formed, sometimes blocking and sometimes peeling off, causing repeated liquid level fluctuations. Figure 1 shows typical copper mould tube liquid level fluctuations caused by nodules.

Figure 1 Liquid level fluctuation caused by nodules and blockage of water inlet

The stopper rod control system causes copper mould tube liquid level fluctuations

The stopper rod is an important tool for flow control in continuous casting. If there is a problem with the stopper rod mechanism, it will inevitably affect the flow control effect and cause liquid level fluctuations. If there is a problem with the fixation of the stopper rod mechanism used in production, or there is a problem with the steel belt, the stopper rod mechanism will be abnormal, resulting in a deterioration of the flow control effect of the stopper rod and large liquid level fluctuations. Figure 2 shows the liquid level fluctuation caused by a typical stopper rod mechanism problem. It is characterized by a sudden large fluctuation. After recovery, the rod position is stable and the same as the previous rod position.

Figure 2 Liquid level fluctuations caused by problems with the stopper rod mechanism

Abnormal erosion of the stopper rod causes fluctuations in copper mould tube liquid level

As casting progresses, especially for calcium-treated steel, the stopper rod is susceptible to melting damage and continuous erosion when working under high-temperature molten steel for a long time, causing abnormal and frequent liquid level fluctuations. In severe cases, the stopper rod may lose control. Figure 3 shows the pouring curve of a certain steel type on site. The liquid level fluctuates frequently during the middle and late stages of pouring, and the rod position continues to decrease. After stopping pouring, the plug rod was lifted out and it was found that the rod head was corroded.

Figure 3 Liquid level fluctuation caused by stopper rod erosion

Crystallizer liquid level fluctuation caused by slag pushing operation

The round billet continuous casting machine of Maanshan Iron and Steel Co., Ltd. uses Cs137 to detect the level of liquid steel in the copper mould tube. It is a non-contact radiographic measurement sensor device. The detected signal of the high-temperature liquid steel level is compared with the set high-temperature liquid steel level value. At the same time, numerical analysis and comparison are performed based on the feedback signals obtained from the actuator sensors. Finally, an output signal that meets the system requirements is generated. The servo transmission mechanism performs relevant actions according to the output signal, controls and adjusts the stopper rod position, and realizes automatic control of the copper mould tube liquid level.

Production practice has found that when the operator pushes the mold slag, the mold slag will interfere with the transmission and absorption of rays, causing the measured liquid level of the copper mould tube to be inconsistent with the actual liquid level. The liquid level fluctuation caused by slag pushing is characterized by liquid level fluctuations from flow I to flow V or from flow V to flow I, and the time interval between each group of fluctuations is about 4 minutes, which is in line with the on-site operator’s habit of pushing slag. And through multiple monitoring and comparison, the time point of each liquid level fluctuation is completely consistent with the time point of each slag pushing, indicating that the slag pushing operation will interfere with the detection of the copper mould tube liquid level.

Copper mould tube liquid level fluctuation caused by crusting on the surface of molten steel in tundish

On-site tracking found that the liquid level fluctuated as shown in Figure 4. The fluctuation lasted for a long time, the fluctuation range was large, and it was a “fish bone” type fluctuation. During the on-site inspection, it was found that the covering agent and the surface of the molten steel were crusted near the plug hole and adhered to the plug rod. When the stopper rod automatically and dynamically adjusts the height according to the liquid level, the stopper rod cannot be adjusted normally due to adhesion or the resistance during adjustment is large and the adjustment is inflexible. This causes the force on the liquid level to rise and fall, causing the liquid level fluctuation to increase, and this kind of regular liquid level fluctuation appears.

Figure 4 The crust on the surface of the molten steel in the tundish causes liquid level fluctuations

Control measures for liquid level fluctuations in round billet continuous casting

Control measures for liquid level fluctuations caused by dummy bar overtension leveling machine

In view of the liquid level fluctuation caused by the dummy bar over-tension leveling machine in a single furnace, the following control measures are taken:

(1) Before single opening, arrange for personnel to clean each joint of the dummy rod to ensure that there is no residual steel residue at the joints and that they can move freely.

(2) Regularly arrange for personnel to replace the chain link between the dummy transition joint and the dummy rod body.

Control measures for liquid level fluctuations caused by nodules and nozzle blockage

Sulfur-containing steel, high-aluminum steel, and non-calcium-treated low-carbon steel are prone to nodulation and nozzle blockage, resulting in liquid level fluctuations. The following measures should be taken:

(1) Protect the whole process of pouring, including whether the sealing ring of the long nozzle of the big bale is damaged, whether the argon sealing pipe of the long nozzle is leaking, whether the middle bale is operated with black slag, and whether the argon flushing pipe of the middle bale is leaking. Prevent secondary oxidation of molten steel from contact with air, causing inclusions such as Al2O3 to accumulate on the rod head, nozzle bowl and nozzle inner wall, causing nodules. Black slag operation should be done in the mold to prevent the molten steel in the mold from being re-oxidized to produce Al2O3 inclusions, or the cold steel will be adsorbed in the mold slag, deteriorating the performance of the mold slag and causing liquid level fluctuations.

(2) For steel types with poor fluidity that are prone to nodulation, use breathable plug rods. The advantages of blowing the argon plug rod: on the one hand, argon gas will form an argon gas film on the head of the plug rod and the nozzle bowl, which is beneficial to promote the floating of inclusions. On the other hand, the blown argon bubbles will enter the inside of the nozzle along with the steel flow. By adjusting the flow state of the molten steel in the nozzle, the accumulation of inclusions such as Al2O3 on the inner wall of the nozzle can be reduced, avoiding the generation of large inclusions and reducing the fluctuation of the copper mould tube liquid level. . However, the strength of the argon flow rate blown by the stopper rod will affect the fluctuation of the copper mould tube liquid level: if the flow rate is too small, the anti-blocking effect will be poor. If it is too large, the liquid level fluctuation in the copper mould tube will increase, and the possibility of secondary oxidation of molten steel and slag entrainment will increase. Therefore, it is necessary to consider these two conflicting factors when selecting the flow rate of argon gas to be blown. The optimal argon gas flow rate for plug rod blowing must be determined to take into account the anti-clogging effect and ensure that the copper mould tube liquid level does not fluctuate.

Control measures for liquid level fluctuations caused by stopper rod control system

In the continuous casting production process, the stable plug rod automatic control system is an important part of the continuous casting machine to achieve stable mold liquid level, constant casting speed, and improvement of slab output and quality.

The stopper rod mechanism should be shielded to eliminate the instantaneous changes in the tracking system of the stopper rod mechanism. Conduct routine inspections on the aviation plugs and seats of the stopper rod mechanism on the on-line middle bag and the spare middle bag. Check the installation status of the stopper rod mechanism in the on-line package to ensure that the locking pin of the plug rod mechanism is installed and tightened and that the air-cooling pipe of the plug rod does not leak.

Control measures for liquid level fluctuations caused by abnormal corrosion of stopper rods

It is necessary to strictly control the quality of stopper rod refractory materials, strengthen the quality inspection of stopper rods, and ensure that the stopper rods have sufficient corrosion resistance and thermal shock resistance.

Control measures for liquid level fluctuations caused by slag pushing

Manually pushing the mold slag will interfere with the detection of the copper mould tube liquid level and cause fluctuations in the copper mould tube liquid level. To this end, it is necessary to:

(1) Refined operation, standardized implementation in strict accordance with the principles of less, frequent and even slag pushing.

(2) Avoid the radioactive source detection channel when pushing slag.

Control measures for liquid level fluctuations caused by crusting on the surface of tundished molten steel

If the covering agent in the tundish is damp or too little is added and the temperature of the casting molten steel is too low, it will cause the surface of the molten steel in the tundish to be encrusted and adhere to the plug rod. The plug rod cannot move freely to control the steel flow, especially in the production of ultra-low carbon alloy steel. The crust on the surface of the molten steel in the tundish is prone to “fishbone” shaped liquid level fluctuations. For this reason, the covering agent must be kept dry, the black slag must be handled during the production process, regular inspections must be made for crusting, and long-term low-temperature casting must be avoided.

Conclusion

Statistics were conducted on 359 furnaces produced before the above measures were taken, and it was found that 10.03% of the liquid level fluctuations were greater than ±3 mm. After taking the above measures, statistics on the production status of 450 furnaces were found, and it was found that the reduction in liquid level fluctuations greater than ±3 mm was 2.18%. Comparison found that after taking measures, the fluctuation of the copper mould tube liquid level was effectively controlled, the quality of the slab was ensured, the rate of downgrading of the slab and the scrap rate were significantly reduced, providing guarantee for stable casting.

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