When and how to add mold flux for continuous casting is the most beneficial?

In the actual production process, the performance of mold powder is closely related to the operation of adding slag. The continuous casting production practice of domestic and foreign steel mills has proved that even if the mold powder is selected correctly, failing to use it correctly will cause no quality problems in the cast slab. When adding protective slag to the mold, the mold slag must be evenly distributed on the molten steel surface in the mold. When the molten steel level fluctuates, drifts, stops casting and other failures occur during the pouring process, corresponding measures must be taken. In the actual operation process, the following items should be paid attention to when adding mold powder.

01 Crystallizer adding slag operation

1. The sequence of adding slag

(1) Prepare slagging tools: the slagging tools are as follows:

Slag removal rod: The slag removal rod is mainly used to remove the large slag circle in the crystallizer. It also has the function of testing the shell to confirm the operation of the shell during bonding. Metallic oxygen tubes or wooden rods can be used. For eddy current liquid level monitoring systems, wooden slag rods must be used. For other liquid level monitoring systems, oxygen-burning pipes can be used, which is convenient and practical.

Slag removal spoon: used to hold the removed slag when changing and removing slag in the crystallizer.

Slag pushing rake: a tool for adding protective slag to the crystallizer. For eddy current type liquid level monitoring systems, wooden wooden speculative rake should also be used to avoid interfering with the signal. Slag removal dustpan: used to hold the removed slag when changing and removing slag in the crystallizer.

(2) Matters needing attention

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

2) Mold powder must not be directly pushed into the corner of the mold.

3) When removing the slag lump or slag ring, pick it up gently to prevent the molten steel from entraining the slag.

4) When the tundish is pouring, the protective slag must be added after the steel surface is immersed in the diverter of the immersion nozzle.

5) Abnormalities are found during the casting of mold powder. If the consumption is too low, the slag must be replaced.

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

02"Black" slag operation

The so-called "black" slag operation is to continuously add enough mold powder to the molten steel surface in the mold to keep the mold powder surface from "leaking red". Generally, the surface of the slag is required to be gray (the surface temperature is about 400°C) instead of red (the surface temperature is about 800°C), and the flame should not 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. It is not suitable to stir the mold powder frequently. This will only have a three-layer structure of the mold powder, which is not conducive to the function of the mold powder.

At present, most of my country's continuous casting adopts manual slag adding method to add mold powder. It should be added frequently, less frequently, and evenly, and the interval time between each addition of mold powder should not be too long. When casting medium-carbon steel and other crack-sensitive steels and using mold powder with a solidification temperature higher than 1100°C, special attention should be paid to the addition of mold powder. When the casting speed is low, the surface layer of mold powder will condense prematurely due to the low heat from molten steel. , It is best to use mold powder with a low solidification temperature with a thickness of 40-50mm to provide more effective thermal insulation.

03 Mold powder operation during pouring

When the continuous casting machine starts pouring, the size of the steel flow in the tundish must be appropriate to ensure that the starter head and the material blocked by the starter head are not damaged. In addition, the emergence time required by the process must be ensured. Pay attention to the operation:

(1) It is customary to try to close the stopper rod 1-2 times to prevent the electric rod during the process of rising the liquid level of the mold steel. 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 mouth), the caster starts the billet drawing and vibration device under the condition of ensuring the emergence time. The starting speed is implemented in accordance with the process regulations, generally 0.3-0.6m/min.

(3) Immediately add protective slag after the immersion nozzle outlet is submerged by molten steel.

When continuous casting is just started, the molten steel in the mold dissipates quickly, and a large amount of heat is absorbed after the mold powder is added, and the molten steel surface may crust. At this time, the slag surface should be gently stirred with a slag removal rod. When crusting, the protective slag can be added in the normal way. Note that the protective slag can only be added when the molten steel is submerged in the side hole of the immersion nozzle.

04 Normal pouring mold powder operation

The mold powder should maintain 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 powder in the mold, the liquid slag layer remains stable and at the same time plays a role of heat preservation.

Under normal pouring, it is forbidden to constantly agitate the crystal liquid level, which will easily damage the melting of the mold powder, so that the performance changes of the mold powder can be grasped at any time. There are two situations of slag entrapment: First, the liquid level fluctuates greatly. When the liquid level drops, a slag ring appears on the wall of the crystallizer and falls toward the center of the crystallizer. If the slag ring is not removed in time, the liquid level may rise. Involve the slag ring to cool the surface unevenly, which may cause steel leakage. Second, the liquid slag layer of mold slag is too thick and solidifies quickly near the mold wall, and a thicker solid layer is concentrated, sticking to the surface of the cast slab. Affect the quality of the cast slab or slow down the heat transfer rate, affect the thickness of the slab shell, and also have the risk of breakout.

05 Mold powder operation when quickly replacing the tundish

(1) After the ladle stops pouring, turn it out of the tundish pouring position.

(2) When the tundish steel liquid level still has a certain depth, appropriately reduce the drawing speed. In addition, the replacement ladle is located at the correct pouring position. At this time, the slag in the crystallizer is gradually removed to facilitate the successful docking.

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

(4) For small and medium section continuous casting billets, control the liquid level at 1/3 of the upper mouth of the mold, and immediately insert the connecting piece into the molten steel in the mold, half of the connecting piece is immersed in the molten steel, and the other half is left outside. Used as a joint. If the climbing is inserted, the climbing is lowered and hooked with a hook to make it fixed to ensure that half of the climbing is exposed to the molten steel surface of the mold, and half is fixed in the new casting billet, and at the same time, the tension leveling is started to reduce the molten steel surface. Set at 2/3 from the upper mouth of the crystallizer.

(5) The new tundish is quickly installed and centered. If a connector is used, the tundish nozzle shall not collide with the connector when it is lowered.

(6) After confirming that the new tundish is properly seated, open the ladle nozzle, pour molten steel into the tundish, and then follow the pouring operation regulations.

06 Mold powder operation at the end of pouring (capping)

The purpose of the capping operation in continuous casting production is to solidify and crust the molten steel at the end of the tail billet to avoid accidents caused by the leakage of molten steel at the end when the mold is pulled out. The capping and closing sequence in production is as follows:

(1) When the molten steel in the ladle is almost finished, no more mold flux is added. The pouring speed is carried out normally until the height of the molten steel in the tundish decreases and the outflow of molten steel becomes slower, at which time the pouring speed decreases accordingly.

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

(3) When the height of the molten steel in the tundish is about 400mm, the mold powder must be removed from the molten steel surface.

(4) The continuous casting machine is switched to the lowest drawing speed and the tundish car is driven away.

After all the slag is removed, the molten steel can be gently stirred with a thin steel rod or an argon blowing pipe. This operation must be uniform and sufficient, and then spray water on the mold copper plate around the end of the casting billet to speed up the top surface It solidifies to form a steel shell.

07 Precautions for using mold powder

(1) The place of production must be protected from moisture. For steel grades with higher quality requirements, pre-melted protective slag should be used.

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

(3) On-site production personnel should be familiar with the technical parameters of the protective slag used on duty, and should be consistent with the type of steel poured.

(4) The production team should designate a person to regularly calculate the slag consumption and measure the thickness of the liquid surface layer during the continuous casting process. Anomalies are found so that measures can be taken.

(5) During the continuous casting production process, the slag must be added frequently and less frequently when adding slag to the mold, and the mold slag must be evenly spread in the mold to ensure the black slag operation, and the surface slag must not be reddened. bright.

(6) On-site personnel should carefully observe the slag condition in the crystallizer. If slag agglomeration is found, adjust it according to specific conditions. If there is a serious slagging circle in the meniscus area, use a special tool to gently tap it. fish out.

08 Parameters that should be measured in the process of using mold powder

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

(2) The relationship between the temperature change of the mold copper plate and its position and time.

(3) The relationship between mold flux consumption and pulling speed.

(4) The accumulation of mold powder in the meniscus area.

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

(6) The relationship between the friction force between the mold and the slab and the pulling speed.

(7) The surface temperature of the cast slab and its changes in the entire length and width directions.

If pouring high-quality cast slabs, the following parameters should also be measured:

(1) The type, frequency and severity of cracks.

(2) The 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 during operation.