What are the categories of continuous casting leakage accidents? What are the main causes?
The so-called leakage refers to the phenomenon that the solidification of the billet shell is not good at the beginning of continuous casting or during the pouring process, or the billet shell is broken or leaked due to other external forces, causing the internal molten steel to flow out. Leakage is one of the malignant accidents in continuous casting production. Severe leakage accidents not only affect the normal production of the continuous casting machine and reduce the operating rate, but also damage the casting machine equipment and cause equipment damage. Leakage accidents are divided into many forms due to different occurrence times and locations on the casting machine. The causes are also different. They are mainly divided into the following points:
(1) Leakage at the beginning of pouring: Leakage is caused by poor start of pouring.
(2) Suspension leakage: The corner gap of the crystallizer is large, the corner pad is concave or the copper plate is scratched, resulting in increased resistance to billet drawing in the crystallizer, which is very likely to cause suspension leakage at the beginning.
(3) Crack leakage: Severe longitudinal cracks, corner cracks or square off occur in the shell of the crystallizer, causing leakage after leaving the crystallizer.
(4) Slag inclusion and steel leakage: Steel leakage is caused by the inclusion of slag or foreign matter in the crystallizer into the local area of the solidified shell, making the shell thickness too thin.
(5) Cutting steel leakage: When the pulling speed is too fast and the secondary cooling water is too weak, the liquid phase hole is too long. After the billet is cut, the central liquid flows out.
(6) Adhesion steel leakage: Steel leakage caused by the billet adhering to the wall of the crystallizer and breaking.
Statistics of a certain factory producing 5 million tons of slabs show that the proportion of various types of steel leakage is: 9.1% at the beginning of pouring, 2.3% at the beginning of slag inclusion, 54.5% at the beginning of bonding, 22.7% at the beginning of cracking, 4.6% at the bulging, 2.3% at the water nozzle, and 4.5% at the others.
What are the reasons for steel leakage during the beginning of pouring? How to prevent it?
The main reasons for steel leakage during the beginning of pouring are as follows:
(1) The cold material in the crystallizer is not placed well, and the ingot head is not plugged tightly.
(2) Starting early, starting the pulling speed is fast, or the pulling speed increases too fast.
To prevent steel leakage during pouring, adequate preparation and inspection should be done before pouring, with the following points of focus:
(1) Check the compactness of the ingot head and the stacking of cold materials;
(2) Check the alignment of the nozzle and the crystallizer;
(3) Check whether there is cold steel on the copper plate of the crystallizer and whether the taper is appropriate;
(4) Check whether the secondary cooling nozzle is unobstructed and intact;
(5) Understand the fluidity of the molten steel, the temperature of the molten steel, the baking state of the tundish and nozzle, and the quality of the protective slag.
(6) The injection size and the residence time of the molten steel in the crystallizer should be determined according to the cross-section of the ingot.
(7) The starting speed is generally maintained at 0.5m/min, and the increase rate should be slow (0.15 m/min) to prevent excessive fluctuations in the crystallizer liquid level.
What are the reasons for steel leakage during casting? How to prevent it?
The fundamental reason for steel leakage during casting is that the local solidified shell of the ingot is too thin after it leaves the crystallizer, and it cannot withstand the static pressure of the molten steel and breaks, resulting in steel leakage.
Therefore, in order to prevent steel leakage during casting, it is necessary to find out the influencing factors of the local thin solidified shell, which mainly include the following aspects:
(1) Equipment factors: The crystallizer is severely damaged and loses its taper, and the ingot is seriously out of square; the arc of the crystallizer and the secondary cooling section are not accurate; the casting strand and the crystallizer are not aligned. In addition, the deformation of the crystallizer copper tube, the serious scratches on the inner wall, the interruption of liquid film lubrication, etc., can also cause the ingot shell to hang and tear.
(2) Process operation factors: such as too fast pulling speed, too high injection temperature, water nozzle misalignment, injection flow deviation, too much fluctuation of the crystallizer liquid level, slag under the injection flow, insufficient cooling strength out of the crystallizer, etc.
(3) Foreign matter or cold steel bites into the solidified shell: If the liquid level fluctuates too much, the unmolten slag in the crystallizer will be drawn into the solidified shell, and the blockage in the water inlet of the tundish will flow into the liquid phase hole of the crystallizer with the steel, and will be captured by the solidification front and cause steel leakage.
In summary, in order to prevent steel leakage during the pouring process, in terms of equipment maintenance, the use of the crystallizer should be checked regularly to ensure the inverted taper of the crystallizer. The crystallizer should be aligned with the secondary cooling guide section to avoid the deformation and rupture of the shell caused by mechanical force during the steel drawing process.
In terms of crystallizer lubrication, the crystallizer should be lubricated evenly to avoid adhesion and hanging leakage caused by poor lubrication between the crystallizer and the shell.
In terms of process operation, attention should be paid to stable operation, reduce the number and amount of changes in the drawing 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 drawn into the crystallizer. For casting with protective slag, protective slag with good molten state and moderate viscosity should be used.
In addition, high-temperature steel with too much overheating should be avoided, because the influence of high-temperature molten steel on steel leakage accidents and ingot quality is quite obvious.
What is bonding leakage and how does it happen?
Bonding leakage is the main form of leakage in the continuous casting process. According to statistics, bonding leakage accounts for more than 50% of the leakage. The so-called bonding is caused by the fluctuation of the liquid level of the crystallizer. There is no liquid slag between the solidified shell of the meniscus and the copper plate. In severe cases, bonding occurs. When the friction resistance increases during billet drawing, the bonding part is broken and expands downward and to both sides, forming a V-shaped rupture line.
When it reaches the outlet of the copper mould tube, leakage occurs.
The occurrence of bonding leakage is as follows: the leakage rate of the inner arc wide surface is higher than that of the outer arc wide surface (about 3:1); bonding leakage is more likely to occur near the middle of the wide surface (about 300mm around the water outlet); large-section slabs are prone to leakage in the middle of the wide surface; and small sections occur in the area close to the narrow surface; aluminum-killed steel has a higher probability of leakage than aluminum-silicon-killed steel; the consumption of protective slag is below 0.25kg/t steel, and the probability of leakage increases.
The reasons for the occurrence of bonding leakage are:
1) The slag ring formed blocks the passage between the inner wall of the copper tube and the shell of the billet;
2) The Al2O3 content of the mold protective slag is high, the viscosity is high, and the liquid surface is crusted, which makes the slag poor in fluidity and difficult to flow into the shell and the copper plate to form a lubricating slag film.
3) High pulling speed under abnormal conditions. Such as high pulling speed when the liquid level fluctuates, and high pulling speed when the molten steel temperature is low.
4) The liquid level of the crystallizer fluctuates too much, such as the immersion nozzle is blocked, the nozzle is seriously biased, and the nozzle condenses when the ladle is replaced, which will cause the liquid level to fluctuate.
What are the countermeasures to prevent bonding leakage?
The countermeasures to prevent bonding leakage during the casting process are:
(1) Monitor the use of protective slag to ensure that the protective slag has good performance. For example, the thickness of the liquid slag layer in the crystallizer is always kept at 8-15mm, the consumption of protective slag is not less than 0.4kg/t steel, and the lumps in the slag are removed in time.
(2) Improve the operating level and control the liquid level fluctuation. (3) Ensure the appropriate pulling speed and the speed variation should be small. The speed increase or decrease range should be 0.15m/min.
