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 copper mould tube 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 squareness are generated in the copper mould tube shell, causing leakage after leaving the crystallizer.
(4) Slag inclusion and leakage: The copper mould tube slag or foreign matter is wrapped into the local area of the solidified shell, making the shell thickness too thin and causing leakage.
(5) Cutting 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 leakage: The billet is adhered to the wall of the crystallizer and pulled off, causing leakage.
Statistics from a factory producing 5 million tons of slabs show that the proportion of various types of leakage is: 9.1% at the beginning of pouring, 2.3% at slag inclusion, 54.5% at adhesion, 22.7% at cracks, 4.6% at bulging, 2.3% at water nozzle solidification, and 4.5% at others.
What are the reasons for leakage during pouring? How to prevent it?
The main reasons for leakage during 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) Determine the injection flow size and the residence time of the molten steel in the crystallizer according to the cross-section of the ingot;
(7) 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 fluctuations in the crystallizer liquid level.
What are the reasons for steel leakage during pouring? How to prevent it?
The fundamental reason for steel leakage during pouring 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 pouring, 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, 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 casting damage during the steel drawing process. The mechanical force will cause the shell to deform and break, causing leakage.
In terms of crystallizer lubrication, the crystallizer should be lubricated evenly to avoid adhesion and leakage of the crystallizer and the shell due to poor lubrication.
In terms of process operation, attention should be paid to stable operation, reducing the number and amount of changes in the drawing speed, keeping the liquid level in the crystallizer stable, and avoiding 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 from being drawn into the product setter. For casting with protective slag, the 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 leakage accidents and ingot quality is quite obvious.
What is bonding leakage and how does it occur?
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 crystallizer, 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 the middle of the wide surface; and small sections occur in the area near 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; 2) the crystallizer protective slag has a high A1203 content, high viscosity, and crust on the liquid surface, 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 crystallizer liquid level 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 liquid level fluctuations.
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 crystallizer liquid slag layer is often 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.
What is the effect of carbon content in molten steel on leakage?
Carbon is a basic element in steel and also the element with the greatest impact on the structure, so the carbon content must be precisely controlled. Steel with C=0.17%~0.22% has a low plastic elongation and a high leakage rate. The probability of leakage in steel with low carbon content is significantly higher than that in steel with high carbon content. The probability of leakage in steel with C≤0.12% is the highest. This is because:
(1) The carbon content in molten steel affects the heat flow in the crystallizer: When the carbon content of the steel is 0.12%, the heat flow transmitted from the crystallizer is the smallest, when the carbon content of the steel is 0.4%, the heat flow transmitted is the largest, and the heat flow transmitted in other carbon content ranges is in the middle.
(2) The carbon content affects the columnar crystal zone: When the carbon content is within the range of ≤0.55%, the columnar crystal zone gradually shrinks, and cracks are easier to slip in the columnar crystal band than in the equiaxed crystal band.
(3) The peritectic reaction of steel has an impact on the solidification cracks of steel: According to the iron-carbon phase diagram, when C20.10%, the transformation of öFe+L→yFe occurs. When the peritectic reaction occurs, at C=0.10%, the initial solidification of ōFe shrinks the most, the segregation of S and P is the smallest, and the shell strength is relatively high; the shell shrinks in the meniscus area, and the shell bends inward, and a small gap appears between the shell and the copper wall, which will cause uneven contact between the rough surface and the copper wall, slowing down the heat transfer, causing the shell growth to slow down and unevenly appearing weak points in local areas, which is the root cause of cracks and leakage. With the increase of carbon content, the amount of vEe generated in the peritectic reaction gradually increases, the solidification shrinkage gradually decreases, the shell maintains good contact with the copper wall of the crystallizer, and the shell grows evenly. Therefore, with the increase of carbon content, the leakage rate gradually decreases. Therefore, the steel with low carbon content has a higher leakage rate than the steel with high carbon content. In production, the carbon content of ordinary carbon steel is best controlled within the range of 0.16%~0.18%, and steel with C≤0.12% should be avoided as much as possible.
What is the effect of “([S]+[P])” in steel on leakage?
The solubility of sulfur in liquid steel is very large, but its solubility in solid steel is very small, and it decreases with decreasing temperature. The precipitated sulfur and iron form sulfide iron, and when solidified, a low-melting eutectic compound with a melting point of only 940°C is produced at the grain boundary. The higher the sulfur content, the more iron sulfide is produced. When iron sulfide crystallizes, it precipitates on the primary grain boundary to form a continuous or discontinuous network structure surrounding the ferrite grains, causing grain boundary brittleness.
Phosphorus is a low-melting eutectic compound with a melting point of only 940°C. Elements that increase the surface tension of molten steel tend to precipitate at the grain boundaries. As the phosphorus content increases, the surface tension of steel decreases a lot, thereby reducing the steel’s resistance to thermal cracking.
The higher the sulfur and phosphorus content in the steel, the higher the probability of steel leakage. Therefore, the sulfur and phosphorus content should be reduced as much as possible in production. The sum of sulfur and phosphorus content is best controlled below 0.040%. This requires strict control of raw materials, careful operation before the furnace, slag with appropriate fluidity and basicity, and appropriate slag volume, good furnace temperature control, and slag blocking operation during steel tapping to prevent slag from falling.
What is the effect of manganese-sulfur ratio on billet leakage?
As the manganese-sulfur ratio increases, the probability of steel leakage gradually decreases. According to solidification theory, when the manganese-sulfur ratio is low, the precipitate contains the majority of low-melting iron sulfide, which is distributed at the grain boundaries, causing grain boundary brittleness and becoming the basis for crack formation. When the manganese-sulfur ratio is high, there is enough manganese to combine with sulfur to form manganese sulfide with a higher melting point. It is dispersed in the austenite matrix in the form of rods and is not easy to form cracks. Therefore, steel with a high manganese-sulfur ratio is not prone to steel leakage accidents.
Modern continuous casting theory and the experience of advanced manufacturers at home and abroad show that the harmful elements in continuous casting molten steel should be reduced as much as possible, and even S and P are required to be ≤0.015%. However, the actual situation of general steel mills cannot meet this requirement, so the manganese content is controlled as much as possible according to the upper and lower limits to ensure that the manganese-sulfur ratio is greater than 15, thereby reducing the steel leakage rate.
