One of the main operational failures encountered in continuous casting is "breakout". When the cast stream slab shell ruptures, the static molten steel in the slab shell overflows and blocks the machine, requiring expensive downtime. In order to pull out the breakout shell, it is necessary to extend the downtime caused by the breakout, because it may block the guide roller or the foot roller, and air cutting is required to clean the blockage and pull out the billet shell. When the temperature of the broken steel billet shell decreases, it needs to be cut into small pieces and taken out of the machine with a straightening machine. The straightening machine is designed to gradually straighten the cold billet shell in the stable stage. The upper roll can provide enough Lift the gravity to create a curved cast stream that is not too long. Therefore, the breakout has a major impact on the effectiveness of the casting machine—influencing productivity and production costs.
Influencing factors of steel breakout The factors that affect the occurrence of steel breakout are:
The temperature and the drawing speed are inconsistent-the higher the degree of molten steel superheat, the thinner the billet shell thickness. Due to the static pressure exerted by the molten steel in the mold, the billet shell expands. When the strength of the blank shell is not enough, breakout is likely to occur. Inconsistent and uneven temperatures have a great influence on the occurrence of steel breakout. When the drawing speed increases, steel breakout is more likely to occur because the mold is not lubricated enough. From the meniscus to the billet shell/mold wall, the mold flux has poor fluidity, and increasing the drawing speed will cause the total heat release cut back. Breakout is often caused by too high drawing speed. When the shell of the blank does not have enough time to solidify to the required thickness, or the metal is too hot, this means that the final solidification happens just below the straightening roll. Due to the stress applied during straightening, the blank The shell tears. When the carbon content in steel is constant, the temperature is high and the drawing speed is fast, which is prone to breakout. Any changes made in the vibration settings will promote the occurrence of steel breakout, because the practice of reducing vibration marks by increasing the vibration frequency will increase the speed of the mold, thereby increasing the friction at the interface.
Poor lubrication between the mold and the billet shell-If poor quality mold powder is used, the molten steel under the meniscus is likely to contain slag, which will cause the mold and the billet shell to bond, and the billet will be interrupted, causing the suspension to break out. During the continuous casting of billet, due to poor or uneven lubrication, the shell of the billet is bonded to the mold, which affects heat transfer and causes bonded breakout.
Ineffective water flow in the mold-reducing the water flow into the mold will result in a decrease in heat transfer, resulting in the formation of a thin shell, which will eventually lead to breakout. The difference in water temperature, pressure and flow rate at the inlet and outlet directly affects the cooling of the crystallizer. The blockage of the mold cooling system leads to an increase in pressure and a decrease in flow rate, which affects heat transfer and is prone to leakage. Therefore, the huge difference in water temperature (high temperature) between the inlet and outlet leads to the adhesion of the mold and the billet shell, which is prone to breakout.
Improper mold geometry-in order to increase the contact surface between the molten steel and the mold, adjust the taper of the mold to adapt to the solidification and shrinkage of the steel, thereby increasing the heat transfer of the mold and increasing the thickness of the billet shell. For the traditional mold with linear taper on the high-speed billet continuous casting machine, the heat transfer at the meniscus quickly solidifies the casting stream into a solid shell. As the shell shrinks, the corners leave the mold and stop heat transfer. Therefore, at the bottom of the mold, except for the remelting at the corners, the green shell continues to grow. When the billet shell leaves the mold, the temperature of the billet shell changes greatly. At this time, increasing the drawing speed may cause breakout. If the adjusted taper does not meet the requirements, there will be an air gap between the mold and the blank shell. When the resistance of the air to the heat transfer in the mold reaches the maximum, it will seriously hinder the formation of the required thickness of the blank shell and eventually lead to breakout . Mold taper loss caused by wear and deformation will cause a significant increase in corner longitudinal cracks, which is the result of corner reheating. As far as the mold deformation is concerned, the reason is that the thickness of the mold copper plate is relatively thin, which is not enough to support the thermal expansion of the copper plate. It is also possible that when the ingot bar is inserted into the mold, the lower part of the mold is damaged and the mold is deformed. Excessive mold taper increases the drawing resistance, resulting in increased mold wear. Inverted taper and heat shrinkage increase the thickness of the air gap, which in turn increases corner wear. Therefore, it is necessary to reduce the heat transfer that increases the surface temperature. This phenomenon is always accompanied by the hydrostatic pressure of the steel, which will induce tensile strain on the corner surface and cause cracks. This kind of crack will greatly reduce the thickness of the blank shell in a fixed manner, which may eventually lead to breakout. The larger the mold fillet radius, the larger the air gap. The air gap hinders heat transfer, resulting in the formation of a thin shell, which is easy to break out. In the slab/bloom continuous caster, 4 separate copper plates are fixed, forming a cavity around them. If there is an air gap at the joint between the two copper plates, the initial metal will penetrate into the air gap and begin to solidify, causing suspension in the later stage, resulting in leakage. Therefore, inappropriate adjustment of the mold will affect the heat transfer mechanism and cause breakout.
The height of the molten steel in the mold is not suitable—during continuous casting, the molten steel in the mold needs to be maintained at 70% to 80% of the height of the mold. If the molten steel level drops below the immersion nozzle, the solidified billet shell formed by the subsequently added molten steel is thinner, which is likely to break out. A drop in the steel level may occur during the water change port, tundish change, or blockage of the tundish water port. When restricting the flow of molten steel from the tundish into the mold, if the drawing speed is not adjusted, steel breakout may occur. Therefore, if the stopper rod is improperly controlled, it will cause the molten steel to overflow and stick to the top of the mold, causing suspension and block drawing, resulting in leakage. The lowering of the molten steel level can also cause slag inclusion. If there is sufficient time for the stopper to close the immersion nozzle, the steel level can be lowered below the allowable limit. If pouring starts again, molten steel will inhibit mold flux and cause slag inclusion. Therefore, when the ladle is changed in full continuous casting, the liquid level of the tundish steel drops. If the operation is improper, the tundish slag can enter the molten steel in the mold through the immersion nozzle. Oxidation products of steel stream, improper deoxidation products, high-viscosity slag formed by improper spraying of aluminum wire in the billet crystallizer, resulting in high Al2O3, may infiltrate the billet shell to form slag inclusions, locally inhibit the formation of the billet shell, and reduce the billet shell The lubricity between the mold and the mold is easy to bond, which causes the blanking to be interrupted and breakout occurs. For the automatic control system of the sizing nozzle, the instability of the molten steel level in the mold will cause the fluctuation of the pulling speed, affect the stable filling of the mold slag between the mold and the blank shell, destroy the continuity of the slag film, and easily make the thickness of the blank shell vary. Uniformity, resulting in surface depression or corner cracking. When the immersion nozzle is replaced in production, the liquid level fluctuates greatly, and it is also easy to cause corner cracks or slag leakage.
Eccentricity of the tundish pouring flow-the eccentricity of the tundish pouring flow causes uneven heat transfer, resulting in uneven thickness of the solidified billet shell, weakening of the strength of the billet shell, and it is difficult to withstand the static pressure of the molten steel, thus leaking. Misalignment of the immersion nozzle, the center of the steel flow is skewed, and the local scouring of the billet shell is severe, and the uniformity of the billet shell cooling in the mold will be greatly affected, and it will also lead to breakout in severe cases. The tundish shell is prone to local deformation for a long time, resulting in misalignment of the nozzle or uneven insertion depth; improper installation of the upper nozzle and seat bricks when repairing the tundish will also cause the misalignment of the immersion nozzle. In the production, it can be judged whether the nozzle is aligned or not by comparing the vibration mark depth on each surface of the breakout billet shell.
The aerosol cooling nozzle is blocked-the foot roll area is located under the crystallizer, where water is directly sprayed on the shell through the nozzle. The blank shell is pressed by the rollers to make the blank shell smoother. At this time, the heat transferred is the largest, which facilitates the formation of a thicker shell. If the nozzle is clogged, the thickness of the blank shell will become thinner, which is likely to cause breakout. In case of blockage, external force needs to be applied by the pulling roller. If the limit is exceeded, the surface of the blank shell will be cracked and the steel will leak.
Irregularity of the starter rod-once the molten steel has solidified above the mold starter rod to form a billet shell of sufficient thickness, the starter rod is slowly pulled out. If the ingot bar is not pulled out regularly, breakout is likely to occur. Similarly, the weak assembly of the ingot bar will cause the molten steel to flow out of the mold, leading to leakage. If the starter bar is separated from the blank shell prematurely before the starter bar head is lifted, it is easy to cause breakout.
Breakout type
According to the appearance of the breakout billet shell, the breakout is roughly divided into the following categories:
Suspension or bonding causes leakage-the molten steel is bonded to the mold, so it is called bonding or suspension. This may be caused by the discomfort of lubrication between the mold and the billet shell or improper adjustment of the mold, and the lubrication discomfort may be caused by poor quality mold powder, slag inclusion in the mold shell, molten steel overflow in the mold, and mold Caused by poor lubrication/unevenness of corner seams and billet continuous casting machines.
Cracks cause breakout-longitudinal cracks at the corners of the billet shell and longitudinal cracks on the wide surface will cause breakout. If the longitudinal cracks cause a breakout, the mold slag will flow unevenly, and the uneven heat transfer of the mold will result in uneven thickness of the blank shell. Improper selection of mold slag and uneven cooling of the mold will cause the blank shell to rupture during cooling. For breakout caused by longitudinal cracks at the corners, the blank shell with insufficient solidification thickness along the narrow surface of the mold will be broken due to tensile stress during shrinkage. Are caused.
Slag breakout-the slab shell entrained with protective slag or large-grain inclusions leads to reduced heat transfer, forming a thin blank shell and breakout. During billet continuous casting, secondary oxidation products, improper deoxidation products in high-viscosity slag during low-carbon steel smelting, and improper aluminum wire spraying in the mold cause high Al2O3, all of which promote the inclusion of slag in the slab and inhibit the growth of the slab , Resulting in a breakout.
Thin shell breakout-Observe that this kind of breakout in the billet continuous caster is caused by the uneven thickness of the shell in the mold. The reason may be the eccentricity of the pouring flow in the mold or the severe deformation of the mold cooling tube.
Stopping pouring causes steel breakout-the continuous casting process is interrupted and fails to be disconnected to stop pouring. If the joint cannot withstand the pulling force exerted by re-casting, the entire furnace will leak. Measures to control breakout Taking into account the impact of breakout on the utilization and effectiveness of the continuous caster, necessary measures must be taken to control the occurrence of breakout.
●Measure the temperature only after argon is blown on the pouring platform to ensure the uniformity of the temperature. According to the chemical composition of steel, the temperature of the pouring stream must be kept overheated at about 60°C before the ladle can be placed on the turntable to ensure that the molten steel is overheated at 25-35°C in the tundish.
●Control the pulling speed according to the temperature monitored in the ladle. When the carbon content in the steel is constant, ensure that the temperature increases as the pulling speed decreases, and the pulling speed increases as the temperature decreases. Therefore, the pulling speed must be adjusted correctly according to the temperature and carbon content of the steel. Gradually increase the drawing speed, and maintain a steady state continuous casting through a certain drawing speed. Any interruption in continuous casting must reduce the drawing speed.
●Any mold powder has an expiration date, so it should not be used after it expires. The mold powder can only be opened during casting and is placed under a high-wattage bulb to dry it. The mold powder in an open bag cannot be used for re-casting. Select the appropriate mold powder according to the specified chemical composition of the steel. At the beginning of casting, an initial mold flux with low viscosity and low melting point is used. For billet continuous casters, ensure that the linseed oil is evenly distributed in the crystallizer.
●For slab/bloom continuous casters, measure the thickness of the slag pool to determine whether the thickness of the slag pool exceeds 10mm and the equipment stroke composed of steel, copper and aluminum wires attached to the steel plate, which helps to avoid slag inclusion , The blank shell is evenly lubricated.
●For the high-speed billet continuous casting machine, a variety of taper crystallizers can be used instead of the traditional linear taper crystallizer. Check the deformation of the mold (if any). Choose the appropriate mold taper and adjust the taper to suit the narrow surface according to the steel rhyme grade and its solidification method on the slab/bloom continuous caster.
●Before the start of continuous casting, check the water flow in the mold by measuring the increase in water pressure to find out the blockage (if any). In general, check the difference in inlet and outlet water temperature, pressure and flow, as well as flow equipment. The water quality should also be checked. According to the grade of steel and its solidification method, adjust the mold cooling mode, that is, the water flow rate (1/min), to adapt to various mold surfaces. In order to control the bonding, a thermocouple is used to detect the temperature change of the mold wall, and the pulling speed is reduced to make the green shell continue to grow uniformly. For a given continuous casting machine, it is necessary to ensure that the difference between inlet and outlet water temperatures cannot exceed the specified value during continuous casting.
●Ensure that the maximum fillet radius along the copper plate is 0.2mm. If corner seams exist at the joints of copper plates, fill the corner seams with plaster or lime before starting continuous casting.
●Install the mold level automatic controller on the continuous casting machine to maintain the mold steel level. In order to distinguish between molten steel and slag in the mold, and to check the slag inclusion, an electromagnetic sensor is installed on the mold.
●Before casting, adjust the tundish nozzle for centering. To deal with the clogging of the nozzle of the tundish, before placing the ladle on the turntable, ensure that the metal wire of the Ca-Si core is sprayed in to meet the requirements of high-aluminum steel in order to form low-melting-point calcium aluminate. Use a freezer to avoid rotation of the stopper.
●Tundish metal protective flux and shielding plate are used between the ladle and the tundish to ensure proper deoxidation products and prevent the formation of secondary oxidation products. For the billet continuous caster, maintain Mn/Si>3.
●Use asbestos rope to seal the starter rod head, and use a chill box to ensure the correct distribution of the chill box before casting.
●To determine the blockage (if any), check the spray cooling nozzle and water flow.
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Analysis on the Causes of Breakout of Continuous Casting Billets
From the billet continuous casting breakout phenomenon, it can be divided into: corner crack breakout, central breakout, pull-off breakout, and start-up breakout. From the analysis of the causes of continuous casting breakout, it can be divided into: 1. Breakout caused by improper operation. 2. The overheating of steel is unreasonable. 3. Breakout caused by mold powder. 4. The mold is not good for arc. 5. The mould vibration frequency and amplitude are unreasonable. 6. The second cold spray water is unreasonable. 7. The mold assembly is unreasonable.
detailed analysis:
01 Reasons for improper operation:
1. The misalignment of the nozzle of the mold causes uneven cooling of the molten steel in the mold, resulting in uneven thickness of the cast slab shell.
2. The steel liquid level is not watched, which causes the liquid steel to be too low to break out or break out of the steel billet after the overflow. Or due to various reasons in the production, the rhythm is unstable, resulting in large fluctuations in the pulling speed, and the solidification curve deviates from the curve of the inner cavity of the copper tube, which is prone to uneven thickness of the blank shell. In the later stage of the use of the crystallized copper tube, cracks in the mold corners are likely to occur. Angle cracking often occurs within a short period of time after the drawing speed is adjusted. Therefore, the drawing speed should be as stable as possible. The drawing speed cannot be adjusted to adapt to the molten steel temperature, smelting cycle and steel supply rhythm. Instead, the molten steel supply should be actively ensured. The quality of molten steel meets the demand of continuous casting; the submerged nozzle has a short life and is frequently replaced. The tundish must be raised as a whole during replacement. If the insertion depth of the nozzle in other flows is too shallow, the liquid level is unstable, which is easy to cause slag and steel leakage. The refractory material of the original nozzle is not matched, and the cold steel is connected between the upper and lower nozzles, and the oxide slag formed by blowing and burning with a small oxygen tube enters the mold, which is easy to cause the leakage of the slag.
3. The slag ring in the crystallizer is not caught in time, causing the casting slab to slag and break out.
4. The nozzle is blocked or mechanically broken, causing leakage.
02 The overheating of steel is unreasonable:
The cracked steel leakage is closely related to the temperature and drawing speed of the tundish, which ensures that the molten steel has a certain degree of overheating and can ensure that the molten steel is poured smoothly. Theoretical studies have shown that for every 10℃ increase in superheat, the thickness of the mold shell at the exit of the mold will decrease by 3%. If the temperature is too high, the mold shell will be thin and the high temperature strength will be low. Once the mold shell is torn by the stress, it will be easy Cracks occur.
03 Breakout caused by mold powder:
1) The mold flux is not added in time, resulting in no lubrication between the casting billet and the copper tube of the mold.
2) The selection of mold powder is unreasonable, that is, the melting point and melting rate are unreasonable.
Mold protection slag function: heat insulation, prevent secondary oxidation, absorb inclusions, lubricate the blank shell and mold copper tube, reduce frictional resistance. There are many varieties of mold powder for continuous casting.
(1) According to the chemical composition of the base department, it can be divided into: SiO2-A12O3-CaO series, SiO2-A1203-FeO, SiO2-A12O3. Na2O system, the former is the most common. On this basis, a small amount of additives (alkali metal or alkali metal oxides, fluorides, borides, etc.) and carbon materials (carbon black, graphite, coke, etc.) that control the melting rate are added.
(2) According to the shape of mold powder, it can be divided into powdery slag (mechanical mixing molding) and granular slag (extruded products are elongated, products formed by disc method are round, and products formed by spray method are hollow. Round particles).
(3) According to the raw materials used, it can be divided into raw material mixed type, semi-pre-melted type and pre-melted type.
(4) According to its use characteristics, according to the characteristics of steel grades, the characteristics of continuous casting equipment and continuous casting process conditions, it can be divided into various specifications of protection (low, medium and high carbon steel mold slag and special steel special slag), heating type opening Pouring slag and so on.
Selection principle of mold flux for continuous casting:
The mold powder of continuous casting mold should have a reasonable melting temperature, melting speed and melting layer structure in the mold; stable and suitable viscosity; sufficient ability to absorb the inclusions in the steel.
04 Due to poor arc alignment, the movement of the mold causes a shearing force on the cast slab to break through.
05 Mould vibration frequency and amplitude.
1. Unreasonable selection of mould vibration frequency and amplitude. The mold has no negative slippage or a small negative slippage causes steel breakout.
2. The vibration imitates the arc difference and the large amount of deflection will produce shear force on the blank shell, affect the lubrication of the mold powder, and increase the drawing resistance. From the perspective of heat transfer, the vibration imitates the arc difference and the large amount of deflection will increase the unevenness of the air gap between the blank shell and the copper tube, resulting in an increase in the difference in the thickness of the blank shell. The general reason is that the steel slag on the east side of the vibrating frame or the damage of the leaf spring causes the vibration to be unstable, the mold deflection is serious, the billet shell wears the two sides of the inner cavity of the copper tube and the inner east corner seriously, and the uneven heat transfer at the corner causes the corner. Cracked steel.
06 Two cold spray water.
The billet shell of the cast slab just out of the mold has a high temperature and loses support. At this time, uniform and strong cooling is required to promote the rapid growth of the billet shell. If the upper part of the secondary cooling is too weakly cooled, reflow will occur, which will cause steel breakout.
07 Crystallizer
1) When the mold is assembled, the uneven water between the water jacket and the copper tube causes uneven cooling of the copper tube, and the heat conduction on the side with small flow rate is low, resulting in a thin cast slab shell.
2) Unreasonable taper of the mold copper tube: 2) The influence of the inverted taper of the inner cavity of the copper tube: The thermal resistance of the mold heat transfer is mainly the air gap. If the air gap is small, the thermal resistance will be small, and if the air gap is large, the thermal resistance will be large. . In the early stage of use of the mold, the inner cavity curve of the copper tube is relatively close to the contraction curve of the blank shell, the air gap is uniform, the heat transfer is uniform, and the thickness of the blank shell is also relatively uniform. During use, the copper pipe is constantly worn and deformed by heat. In the middle and late stages of use, the total taper becomes smaller, and the heat transfer under the meniscus is large, and the copper tube is locally deformed, which also increases the unevenness of the blank shell.
The billet shell is prone to bulging in the lower part of the mold. Sampling shows that the bulge of 150 mm×150 mm cast billet is more than 2 mm prone to deviation angle internal cracking, and the billet shell loses support after exiting the mold, which is prone to breakout. The influence of the inner surface of the copper pipe: During the use of the copper pipe, excessive and skewed cold steel was placed due to the breakout accident, which caused scratches on the face and corners with a depth of more than 1 mm. During the drawing process, the blanks were scratched. The thermal resistance between the shell and the copper tube wall is large, the billet shell is thin, and dents are prone to appear, and there are obvious cracks at the bottom of the dents. At this time, if the overheating degree increases or the pulling speed is suddenly increased, it is easy to leak steel at the cracks. The quality of the copper pipe is poor, especially the partial coating on the face and corners of the copper pipe falls off. Increased thermal resistance, resulting in unstable heat transfer, and easy to cause leakage. In addition, there are trachoma in the copper tube blank. As the amount of steel passing through the mold increases, the inner surface of the copper tube is worn out, the trachoma leaks, and the phenomenon of steel hanging occurs, which may cause cracks and leaks in severe cases.
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