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Types, causes and preventive measures of continuous casting breakout accidents

[vc_row][vc_column][vc_column_text]The so-called breakout refers to the phenomenon that the slab shell is not solidified in the initial stage of continuous casting or during the pouring process, or the slab shell is broken or leaked due to other external forces, which causes the internal molten steel to flow out. Breakout is one of the vicious accidents in continuous casting production. Serious breakout accidents not only affect the normal production of the continuous casting machine and reduce the operation rate, but also damage the equipment of the casting machine and cause equipment damage. Breakout accidents can be divided into various forms due to the different time of occurrence and the different positions on the casting machine, and the causes are also different, mainly divided into the following points:

⑴ Breakout of open pouring: Breakout caused by poor start of pouring.

⑵Hanging breakout: large corner seams of the mold, recessed corner pads or scratches on the copper plate, resulting in increased resistance to drawing blanks in the mold, and easy to start hanging breakout.

⑶ Crack breakout: severe longitudinal cracks, corner cracks or detachment occur in the mold shell, which will cause breakout after exiting the mold.

⑷Bleakage with slag inclusion: due to mold slag or foreign matter wrapped in a local area of the solidified shell, the thickness of the blank shell is too thin to cause a breakout.

⑸ Cut the breakout: When the drawing speed is too fast and the secondary cooling water is too weak, the liquid phase cavity is too long, and the center liquid will flow out after the slab is cut.

⑹ Bonded breakout: The breakout caused by the casting billet adhered to the mold wall and broken.

continuous casting breakout accidents

  • Open pouring breakout

(1) The head of the middle package stopper and the bowl of the upper nozzle are not properly baked. Because the bowl is relatively low, the baking flame of the traditional baking method cannot reach the bowl, causing the temperature of the bowl to be 100-200°C lower than other parts. The low temperature of molten steel can easily cause cold steel pads and steel loss control, which is forced to increase the drawing speed, resulting in insufficient solidification of the billet head, resulting in breakout.

(2) The paper rope is loose and the molten steel leaks from the gap; the paper rope is damp, and explodes after meeting the molten steel to create a gap, and the molten steel leaks from the gap.

(3) The iron filings layer is too thin, causing molten steel to ooze from the gap after burning the paper rope; the iron filings layer is too thick, which will lead to insufficient strength of the billet head and the shell of the billet to be broken; the iron filings are damp, oily or have debris , It explodes or burns when it encounters molten steel, the molten steel oozes out from the gap after burning the paper rope or the billet head is not strong enough, and the billet shell is broken.

(4) The traditional method of plugging the starter is to use paper rope to close the gap between the starter head and the mold. When the molten steel arrives at the station to measure the temperature, first evenly spread 20-30mm thick iron nail chips on the starter head, and then place the steel slats crosswise on the iron nail chips. If the steel strips are not placed well, the molten steel will directly wash away the iron filings and paper rope; if the steel strips are not fully melted, the primary billet shell will be too thin, which will cause the billet shell to be torn during drawing.

(5) The following problems exist in the operation: the flow of pouring steel is too large, the iron filings are scattered or the molten steel is splashed on the crystal wall, and the corner joints to form steel; the starting speed is too fast, each time exceeding 0.1m/min, The primary shell cannot withstand the pulling force; foreign matter enters the mold and bites into the primary shell.

(6) The following problems exist in the equipment: the starter slides more than 20mm; the corner seam of the mold is too large, exceeding 0.30mm.

In order to prevent steel leakage during pouring, adequate preparation and inspection should be made before pouring, and the following points should be paid attention to:

⑴Check the compactness of the starter head and the stacking of cold material;

⑵Check the alignment of the nozzle and the crystallizer;

⑶ Check whether there is cold steel on the copper plate of the mold and whether the taper is suitable;

⑷Check whether the secondary cooling nozzle is unblocked and intact;

⑸ Understand the fluidity of molten steel, the temperature state of molten steel, the state of baking of the tundish and nozzle, and the quality of the protective slag. The upper nozzle and the immersion nozzle are fully baked to ensure that the temperature of the immersion nozzle is above 800 degrees within 2.5 hours, and the temperature of the upper nozzle stopper is above 1100 degrees.

⑹The injection size and the residence time of molten steel in the mold should be determined according to the section of the cast slab.

⑺ The starting speed is generally maintained at 0.5m/min, and the increase speed should be slow (0.15m/min) to prevent excessive fluctuations of the crystallizer liquid level.

  • Pouring breakout

The root cause of the breakout during the pouring process is that the partially solidified shell of the cast billet after exiting the mold is too thin, which cannot withstand the static pressure of the molten steel and breaks and causes the breakout. Therefore, in order to prevent the breakout accident during the pouring process, it is necessary to find out the influencing factors of the partial thinness of the solidified shell, which mainly include the following aspects:

⑴Equipment factors: the mold is severely damaged and loses its taper, and the casting billet is seriously de-squared; the mold and the secondary cooling section are not aligned correctly; the casting flow and the mold are not aligned properly. In addition, the mold copper tube is deformed, the inner wall is severely scratched, and the liquid film lubrication is interrupted, which will also cause the blank shell to hang and tear.

⑵ Process operation factors: such as too fast drawing speed, too high injection temperature, misalignment of nozzle, skewed injection, too large fluctuation of crystallizer liquid level, injection of slag, insufficient cooling intensity of mold outlet, etc.

⑶ Foreign matter or cold steel bites into the solidified shell: If the liquid level fluctuates too much, the unmelted slag block in the mold is caught in the solidified shell, and the blockage in the tundish nozzle flows with the steel to the liquid cavity of the mold and is captured by the solidification front And lead to breakout.

To sum up, in order to prevent steel leakage during the casting process, in terms of equipment maintenance, the use of the mold should be checked regularly to ensure the inverted taper of the mold. The mold should be centered with the secondary cooling guide section to avoid the casting billet In the process of drawing steel, the billet shell deforms and cracks due to the mechanical force, which causes the leakage.

In terms of mold lubrication, the mold should be uniformly lubricated to avoid the adhesion between the mold and the billet shell and the suspension leakage due to poor lubrication.

In terms of process operation, attention should be paid to stable operation, reduce the number and amount of changes in the pulling speed, keep the liquid level in the crystallizer stable, and avoid excessive or frequent fluctuations. At the same time, the liquid level in the tundish should not be too low to avoid a large amount of non-metallic inclusions or steel slag being caught in the mold. For casting with mold powder, mold powder with good melting state and moderate viscosity should be used. In addition, high-temperature steel with too much overheating should be avoided, because the impact of high-temperature molten steel on breakout accidents and slab quality is quite obvious.

  • Bonded breakout

Bonded breakout is the main form of breakout in the continuous casting production process. According to statistics, bonded breakout accounts for more than 50% of many breakouts. The so-called adhesion is caused by the fluctuation of the crystallizer liquid level, and there is no liquid slag between the solidified shell of the meniscus and the copper plate, and adhesion occurs in severe cases. When the billet is drawn, the frictional resistance increases, the bond is broken, and expands downward and on both sides, forming a V-shaped rupture line, and breakout occurs when it reaches the mouth of the mold.

Bonded breakout occurs in the following situations: the incidence of breakout on the wide surface of the inner arc is higher than that of the wide surface of the outer arc (about 3: 1); near the middle of the wide surface (about 300mm around the nozzle) is more likely to occur. Broken slabs are prone to breakout in the middle of the wide face; while small sections occur in the area close to the narrow face; aluminum-killed steel has a higher probability of breakout than aluminum-silicon killed steel; mold slag consumption is below 0.25kg/t steel, and leakage Increased steel probability.

The reasons for bond breakout are:

1) The formed slag ring blocks the passage of liquid slag into the inner wall of the copper tube and the blank shell;

2) Mold mold powder has high Al2O3 content, high viscosity, and liquid surface crusts, which make the slag fluidity poor, and it is not easy to flow between the blank shell and the copper plate to form a lubricating slag film.

3) High pulling speed under abnormal conditions. Such as the high drawing speed when the liquid level fluctuates, and the high drawing speed when the molten steel temperature is low.

4) Excessive fluctuations in the liquid level of the mold, such as blockage of the immersion nozzle, serious flow of the nozzle, condensation of the nozzle when replacing the ladle, etc. will cause the liquid level fluctuation.

The countermeasures to prevent bond breakout during the pouring process are:

(1) Monitor the use of mold powder to ensure that the mold powder has good performance. For example, the thickness of the liquid slag layer of the measuring mold is always kept at 8-15mm, the consumption of mold slag is not less than 0.4kg/t steel, and the agglomeration in the slag is removed in time.

(2) Improve operation level and control liquid level fluctuation.

(3) Ensure proper pulling speed, and the pulling speed change range should be small. The suitable lifting speed range is 0.15m/min.

(4) Strictly control the quality of molten steel, improve the cleanliness of molten steel, and reduce inclusions in steel.

(5) Strengthen the inspection of the copper plate of the mold. If there are cracks or other factors that affect the flatness of the copper plate, it must be polished. If the problem is serious, it must be offline.

(6) Adhere to the automatic control system of the crystallizer liquid level. When the automatic control of the liquid level is unstable, switch to manual pouring in time to ensure that the liquid level fluctuation is within the minimum range and the fluctuation target value is controlled within ±3mm

(7) Reduce the temperature of molten steel in the tundish, control the superheat of molten steel within 35℃, and prevent high temperature steel casting.

(8) The adhesion prediction system is used to report the occurrence of adhesion in advance, especially for preventing adhesion breakout. The bonding prediction system monitors the temperature through a thermocouple embedded in the mold. When a cold spot or a hot spot on the meniscus moves down in the thermal image, it is considered that the green shell is bonded.

continuous casting breakout accidents

  • Suspension breakout

The molten steel is drilled into the corner seams of the mold to form cold steel or the hanging steel is connected to the primary billet shell at the four corner seams of the upper mouth of the mold, which causes the drawing to be blocked and causes the breakout. The corner seam inspection system of the mold should be strictly inspected. When the corner seam is larger than 0.35mm, a new mold should be replaced in time; the upper corner seams of the mold should be plugged with asbestos to prevent steel from hanging. Once found, remove it immediately.

  • Slag entrapment/slag breakout

Slag breakout is caused by mold protection slag, molten steel inclusions, and refractory materials falling off from ladle and tundish, etc.; after the breakout, obvious knots can generally be seen at the part of the residual blank breakout. Scum.

1) Involvement of mold flux breakout When the design of SEN is unreasonable, SEN insertion is too shallow, poor alignment of SEN or perforation and cracking of SEN will cause mold flux to be involved. In addition, improper operation of adding mold powder and slag removal will cause slag entrapment and breakout.

2) The molten steel inclusions are involved in the breakout. When the refined molten steel is poorly deoxidized, the soft blowing time is not enough, the S content of the molten steel is high before the calcium treatment (more than 0.01%), and the Ca/Al (less than 0.09) and Ca/AlOxy( Less than 1) will cause the Al2O3 and CaS inclusions in the molten steel to be higher. In addition, the ladle drainage will protect the inclusions caused by improper pouring. If the tundish does not float up sufficiently, the stopper rod will block the water inlet and cause the liquid Surface fluctuations, when the stopper rod or inclusions accumulated in SEN are suddenly washed down, it may cause slag entrapment and breakout. In addition, when the tundish covering agent is incorrectly added, too much slag under the ladle, and the tundish liquid level is too low, it will lead to slag entrapment and leakage.

3) Involvement of refractory materials in the breakout ladle working lining refractory material, the falling off of the refractory material of the working layer of the tundish, the SEN or stopper rod falling off and being involved in the mold, it is very likely to cause the breakout casting process to easily cause casting Slag inclusion and treatment measures and treatment measures:

In the pouring process, it is easy to cause slag inclusion and treatment measures:

1) Tundish drop level: When the tundish drop level is less than or equal to 35 tons, the drawing speed should be reduced to the minimum drawing speed required by the process in time before the machine. When the tundish tonnage returns to the normal liquid level of more than 40 tons, the process is normal Speed up operation. Avoid excessively high pulling speed or premature increase of speed causing crystallizer slag entrapment. (The lowering of the liquid level in the tundish can also easily cause fluctuations in the liquid level. Check whether there is slag around the crystallizer in time)

2) Nozzle drift or side turn: When nozzle drift occurs, first control the pulling speed ≤1.2m/min (section ≤1200mm) 4≤1.0m/min (section ≥1200mm) in front of the machine, and focus on side turning when pushing slag The side is guaranteed not to expose the steel flower. At the same time, check whether the stopper mechanism and the opening degree are normal. The water port can be changed if the rollover is serious.

3) The nozzle insertion depth is inappropriate: When the immersion nozzle insertion depth is too shallow and the drawing speed is low, the stream will not impact the narrow surface of the mold, and the stream will flow back to the side of the nozzle, and its downward speed will protect The slag is involved in molten steel, is captured by the nozzle stream, and enters the mold to cause slag entrapment. If the nozzle immersion depth is too deep, it is easy to reflux and entrain slag; if it is too deep, the chance of inclusions and bubbles being caught in the depth of the cast slab is increased, and the hot spot moves downward, which increases the probability of steel breakout and causes poor slag removal. Bad lubrication. Therefore, the pouring steel can track the entry depth of the nozzle at any time and adjust it in time. (Depth of nozzle immersion 110-180mm)

4) Pulling speed control: the drawing speed is faster, the melting structure of the mold powder is changed, the thickness of the molten slag layer becomes thinner, and the probability of the powder slag layer being wrapped in the molten steel increases; the drawing speed is too slow, and it is easy to cause reflux and slag. High precision Grab the molten steel at drawing speed, control the production rhythm and reduce the low drawing speed pouring.

5) Mold slag slag control: ensure that the thickness of the mold slag meets the process requirements (8-15mm). When the thickness of the mold slag is less than 6mm, the liquid slag’s ability to absorb inclusions in the molten steel is weakened and it is easy to cause slag inclusion in the casting slab. Steelworkers must pay attention to the use of mold powder and replace mold powder in a timely manner when abnormalities are found.[/vc_column_text][/vc_column][/vc_row]

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