When and how to add mold flux for continuous casting?

　　In the actual production process, the performance of mold flux is closely related to the operation of adding slag. The continuous casting production practice in steel plants at home and abroad has proved that even if the mold flux is selected correctly, it will cause no quality problems if it is not used correctly. When adding mold flux to the mold, the mold flux must be evenly distributed on the steel surface in the mold. In case of liquid steel level fluctuation, eccentric flow, stop casting and other faults during pouring, corresponding measures must be taken. During actual operation, the following items shall be paid attention to when adding protective slag.
 

　　1. Slag feeding operation of crystallizer

　　1.1 slag adding operation sequence

　　(1) Prepare the slag adding tools: the slag adding tools are as follows: slag fishing rod: the slag fishing rod is mainly used to remove the large slag circle in the crystallizer, and to test the blank shell to confirm the operation of the blank shell during bonding. Metal oxygen burning tube or wood strip can be used. For eddy current liquid level monitoring systems, wooden slag fishing rods must be used. For other liquid level monitoring systems, oxygen burning tubes can be used, which is convenient and practical. Slag scoop: it is used to hold the salvaged slag when changing and salvaging the slag in the crystallizer. Slag pushing rake: a tool for adding protective slag to the crystallizer. For eddy current liquid level monitoring system, wooden speculative rake shall also be used to avoid signal interference. Slag dustpan: it is used to hold the salvaged slag when changing and salvaging the slag in the crystallizer.

　　(2) Precautions

　　1) When the liquid level of the crystallizer fluctuates abnormally, the mold flux shall be added after the liquid level is stable.

　　2) The protective slag shall not be directly pushed to the corners of the crystallizer.

　　3) When the slag block or slag ring is removed, it shall be pulled gently and slowly to avoid slag entrapment by molten steel.

　　4) During tundish pouring, the protective slag must be added after the steel level is immersed in the diversion port of the submerged nozzle.

　　5) If the mold protective slag is found abnormal during pouring, if the consumption is too low, it must be replaced.

　　6) It is better to add slag frequently and thinly to ensure that a layer of new slag is always covered on the molten slag layer.
 

　　2. "Black" slag operation

　　The so-called "black" slag operation is to continuously add enough protective slag to the steel level in the crystallizer to keep the surface of protective slag from "red leakage". Generally, the slag surface is required to be gray (the surface temperature is about 400 ℃) rather than red (the surface temperature is about 800 ℃), and the escaping flame is not required to be too high, about 20mm. Generally, the thickness of the liquid slag layer is 7-15mm, and the thickness of the original slag layer is 15-20mm, which is more appropriate. It is not suitable to mix the protective slag frequently. This will only affect the three-layer structure of the protective slag, which is not conducive to the role of the protective slag.

　　At present, the mold flux is mostly added manually in continuous casting in China. It should be added frequently, less and evenly. The interval between adding mold flux each time should not be too long. When casting medium carbon steel and other crack sensitive section steels with mold flux with solidification temperature higher than 1100 ℃, special attention shall be paid to the addition of mold flux. When casting speed is low, due to low heat from molten steel, the surface layer of mold flux will condense prematurely. It is best to use mold flux with low solidification temperature and thickness of 40-50mm to provide more effective thermal insulation.
 

　　3. Protective slag operation during pouring

　　During the start-up of continuous casting machine, the steel flow of tundish must be appropriate to ensure that the ingot head is not damaged and the materials blocked by the ingot head are not overturned. In addition, the seedling emergence time required by the process shall be ensured. During operation, attention shall be paid to:

　　(1) During the rising of the steel level in the crystallizer, it is customary to try to close the stopper for 1-2 times to prevent the electric rod. Ensure that the emergence time is between 30-90s.

　　(2) When the molten steel in the mold reaches the specified height (generally 70-100mm from the upper opening), the casting machine starts the blank drawing and vibration device under the condition of ensuring the emergence time. The starting speed shall comply with the process regulations, generally 0.3-0.6m/min.

　　(3) Add protective slag immediately after submerged nozzle outlet is submerged by molten steel. When the continuous casting is just started, the molten steel in the mold dissipates heat quickly, and a large amount of heat is absorbed after adding the protective slag. The steel liquid level may be crusted. At this time, the slag surface should be stirred gently with a slag fishing rod. When it is proved that there is no crusting, the protective slag can be added in the normal way. Note that the protective slag can be added only when the molten steel submerges the side hole of the submerged nozzle.
 

　　4. Normal pouring of protective slag

　　The mold flux shall be kept at a certain thickness in the mold, and the thick slab is usually controlled within the range of 40-50mm. In order to ensure the uniform melting of the mold flux in the mold, keep the liquid slag layer stable and play the role of heat preservation. Under normal pouring, it is forbidden to frequently stir the crystallization liquid level, which is easy to damage the melting of the protective slag, so as to keep abreast of the performance changes of the protective slag at any time. There are two cases of slag entrapment: first, the liquid level fluctuates greatly. When the liquid level drops, a slag circle appears near the crystallizer wall and falls to the center of the crystallizer. If the slag circle is not removed in time, when the liquid level rises, the slag circle may be involved, resulting in uneven cooling of the surface and the risk of steel leakage; Second, the liquid slag layer of the protective slag is too thick and solidifies quickly near the mold wall. A thicker solid layer is concentrated and adhered to the slab surface, which affects the slab quality or slows down the heat transfer rate, affects the slab shell thickness, and also has the risk of steel leakage.
 

　　5. Operation of protective slag during rapid replacement of tundish

　　(1) After the ladle pouring is stopped, transfer it out of the tundish pouring position.

　　(2) When the tundish steel level is still at a certain depth, the billet drawing speed shall be properly reduced. In addition, the replacement ladle will be in the correct pouring position. At this time, gradually remove the slag in the crystallizer to facilitate the successful docking.

　　(3) When the liquid level of tundish drops to about 400mm (be careful not to let the slag enter the crystallizer), stop pouring immediately, drive away the tundish truck quickly, and appropriately reduce the secondary cooling water.

　　(4) For medium and small section continuous casting billets, control the liquid level at 1/3 of the upper opening of the mold, and immediately insert the connector into the molten steel in the mold. Half of the connector is immersed in the molten steel, and the other half is left outside as a joint. If after inserting the connecting plate, the connecting plate descends, hook it with a hook to make it fixed, so as to ensure that half of the connecting plate is exposed to the steel liquid level of the crystallizer and half is fixed in the new slab. At the same time, start the tension straightening to set the steel liquid level at 2/3 from the upper opening of the crystallizer.

　　(5) The new tundish is quickly put in place and centered. If connecting pieces are used, the tundish nozzle shall not collide with the connecting pieces when it is lowered.

　　(6) After the new tundish is confirmed to be correct, open the ladle nozzle, inject molten steel into the tundish, and then follow the pouring operation regulations.
 

　　6. Protective slag operation at the end of pouring (capping)

　　The capping operation of continuous casting production aims to solidify the molten steel at the end of the tailstock to avoid accidents caused by the leakage of molten steel at the end when pulling out the mold. The capping and ending sequence in production is as follows:

　　(1) When the molten steel of the ladle is almost poured, the protective slag is no longer added. The pouring speed shall be normal until the liquid level of molten steel in tundish decreases and the outflow of molten steel slows down. At this time, the pouring speed shall be reduced accordingly.

　　(2) When about 1/2 of the molten steel in the tundish is poured, the pouring speed shall be reduced to about 1/2 of the normal pouring speed.

　　(3) When the height of molten steel in tundish is about 400mm, the protective slag must be removed from the steel level.

　　(4) Turn the continuous casting machine to the minimum speed and drive the tundish car away.

　　(5) After all the slag is removed, the liquid steel can be stirred gently with a fine steel bar or argon blowing pipe. This operation should be uniform and sufficient, and then spray water on the copper plate of the mold around the end of the slab to accelerate the top surface solidification to form a steel shell.
 

　　7. Precautions for using mold flux

　　(1) The place of storage shall be moisture-proof. For steel with high quality requirements, premelted protective slag shall be used.

　　(2) When using the protective slag stored for more than half a year, it shall be dried and tested for moisture content before use.

　　(3) The on-site production personnel shall be familiar with the technical parameters of the protective slag used on duty, which shall be consistent with the cast steel.

　　(4) The production team shall designate special personnel to regularly calculate the slag consumption and measure the liquid level layer thickness during continuous casting. Find out the abnormality so as to take measures.

　　(5) In the process of continuous casting production, when adding slag to the crystallizer, it must be added frequently and less, and the protective slag must be evenly spread in the crystallizer to ensure the black slag operation, and the slag color on the surface must not become red and bright.

　　(6) The on-site personnel shall carefully observe the slag condition in the crystallizer. In case of slag agglomeration, it shall be adjusted according to the specific conditions. In case of serious slagging in the meniscus area, it can be gently knocked out with special tools.
 

　　8. Parameters to be measured during the use of mold flux

　　(1) Measure the total heat flow value in the crystallizer.

　　(2) The temperature change of the copper plate in the mold is related to its position and time.

　　(3) Relationship between mold flux consumption and casting speed.

　　(4) Agglomeration of mold flux in meniscus area.

　　(5) Measure the vibration marks of the slab in the meniscus area.

　　(6) Relationship between friction force and casting speed between mold and slab.

　　(7) Slab surface temperature and its variation in the whole length and width direction. If casting high-quality billets, the following parameters shall be measured: (1) type, frequency and severity of cracks. (2) Frequency of visible oxide defects. The relationship between the type, frequency, size and severity of subcutaneous inclusions and the operating variables of the continuous casting machine.