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Apr 1, 2021
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What are the types of continuous casting breakout accidents?

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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, causing 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, reduce the operating 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 and location on the casting machine, and the causes are also different, mainly divided into the following points:

⑴ Breakout when pouring starts: poor start-up when pouring starts to break out.

⑵ Suspension 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 Crackle breakout: Serious longitudinal cracks, corner cracks or detachment occur in the mold shell, which will cause a breakout after exiting the mold.

⑷ Breakout of slag inclusion: Because the slag or foreign body of the mold is wrapped in the local area of ​​the solidified shell, the thickness of the blank shell is too thin, which causes the breakout.

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

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

Slab

The statistics of a factory producing 5 million tons of slabs show that the proportions of various types of breakouts: 9.1% for pouring, 2.3% for slag inclusion, 54.5% for bonding, 22.7% for cracks, 4.6% for bulge, and 2.3% for nozzle condensed steel. Other 4.5%.

What are the reasons for steel breakout during pouring? How to prevent it?

The main reasons for steel breakout during pouring are as follows:

⑴ The cold material in the crystallizer is not properly placed, and the starter head is not plugged.

⑵ Start early, start pulling fast, or pulling speed increases too fast.

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

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

⑵ Check 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;

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

⑸ Understand the fluidity of molten steel, the temperature state of molten steel, the baking state of the tundish and the nozzle, and the quality of the protective slag.

⑹ The 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 pulling speed is generally maintained at 0.5m/min, and the increase speed should be slow (0.15 m/min) to prevent excessive fluctuation of the crystallizer liquid level.

What are the reasons for steel breakout during the pouring process? How to prevent it?

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

⑴ Equipment factors: the mold is severely damaged and loses its taper, and the cast billet is severely de-squared; the mold and the secondary cooling section are inaccurate in the arc; 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 may 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 crystallizer, 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 engulfed 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 leakage of steel 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, and the mold should be aligned with the secondary cooling guide section to prevent the casting slab from being 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, it should be ensured that the mold is evenly 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 a good molten 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.

What is bonded breakout and how does it happen?

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.

The occurrence of bond breakout has the following conditions:

The incidence of breakout in the wide surface of the inner arc is higher than that of the wide surface of the outer arc (approximately 3:1); near the middle of the wide surface (about 300mm around the nozzle) is more prone to bond breakout; large section slabs are prone to breakout in the middle of the wide surface ; The small section occurs near the narrow surface; aluminum-killed steel has a higher rate of breakout than aluminum-silicon killed steel; the mold slag consumption is below 0.25kg/t steel, and the probability of breakout increases.

The reasons for bond breakout are:

1) The formed slag ring blocks the passage between the liquid slag entering 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) Crystallizer The liquid level fluctuates too much, such as the blockage of the immersion nozzle, serious flow of the nozzle, and condensation of the nozzle when replacing the ladle will cause the liquid level to fluctuate.

What are the countermeasures to prevent cohesive breakout?

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

(1) Monitor the use status of the 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 fluctuations.

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

#Continuous casting

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