One of the main operating faults encountered in continuous casting is “breakouts”. When the strand shell breaks, the molten steel that is stationary in the shell overflows and blocks the machine, requiring expensive downtime. In order to pull out the broken shell, the downtime caused by the breakout is extended, because it may block the guide rollers or foot rollers, and it is necessary to use gas cutting to clear the blockage and pull out the shell. When the temperature of the broken shell decreases, it needs to be cut into small pieces and removed from the machine by a straightener, which is designed to gradually straighten the curved cold shell during the stable phase. The upper rolling rollers provide sufficient lifting gravity to make the curved strands not too long. Therefore, breakouts have a significant impact on the effectiveness of the casting machine – affecting productivity and production costs.
Factors affecting breakouts Factors affecting the occurrence of steel leakage
Inconsistency in temperature and pulling speed
The higher the superheat of the molten steel, the thinner the shell thickness. Due to the static pressure exerted by the molten steel in the crystallizer, the shell expands. Breakouts are prone to occur when the shell strength is insufficient. Inconsistent and uneven temperatures have a great impact on the occurrence of breakouts. Breakouts are more likely to occur when the pull speed is increased because the mold is not lubricated enough, the mold powder has poor flow from the meniscus to the shell/mold wall, and increasing the pull speed will result in a decrease in the total heat release. Breakouts are often caused by too high a pull speed, when the shell has not had enough time to solidify to the required thickness, or the metal is too hot, which means that the final solidification occurs just below the straightening rolls and the shell tears due to the stress applied during straightening. For a given carbon content in the steel, high temperatures and fast pull speeds are prone to breakouts. Any change in the vibration settings will promote breakouts because the attempt to reduce vibration marks by increasing the vibration frequency will increase the mold velocity and thus increase the friction at the interface.
Poor lubrication between the crystallizer and the shell
If poor quality protective slag is used, the molten steel below the meniscus is prone to slag inclusion, resulting in adhesion between the crystallizer and the shell, interruption of billet drawing, and hanging steel leakage. During continuous casting of square billets, due to poor or uneven lubrication, the shell adheres to the crystallizer, affecting heat transfer and causing bonding steel leakage.
Incorrect way of adding protective slag
Due to the operating habits of on-site workers, too much is added at one time, and it is mainly concentrated on the inner arc, which is sloped, which will cause uneven filling of liquid slag, affecting lubrication and uniform heat transfer between the crystallizer and the shell. Under normal casting conditions, it is not necessary to remove small slag strips, and it should be forbidden to use slag rods to test whether slag strips are formed in the crystallizer, which will destroy the uniform formation of the initial shell of the meniscus.
Ineffective water flow in the copper mould tube
Reducing the water flow entering the crystallizer will lead to reduced heat transfer, resulting in the formation of a thin shell, and ultimately steel leakage. The difference in water temperature, pressure and flow rate at the inlet and outlet directly affects the cooling of the crystallizer. Blockage of the crystallizer cooling system leads to increased pressure and reduced flow rate, which affects heat transfer and is prone to steel leakage. Therefore, the huge difference in inlet and outlet water temperature (high temperature) causes the crystallizer to stick to the billet shell, which is prone to breakage and steel leakage.
Improper crystallizer geometry
In order to increase the contact surface between the molten steel and the crystallizer, the crystallizer taper is adjusted to adapt to the solidification shrinkage of the steel, thereby increasing the heat transfer of the crystallizer and increasing the thickness of the billet shell. For the traditional crystallizer with linear taper on the high-speed billet continuous casting machine, the heat transfer at the meniscus quickly solidifies the cast strand into a solid shell. As the shell shrinks, the corners detach from the crystallizer and stop heat transfer. Therefore, at the bottom of the crystallizer, except for the remelting of the corners, the billet shell continues to grow. When the billet shell leaves the crystallizer, the temperature of the billet shell changes greatly. At this time, increasing the pulling speed may cause steel leakage. If the adjusted taper is not up to standard, an air gap will be generated between the crystallizer and the billet shell. When the air resistance to heat transfer in the crystallizer reaches the maximum, it will seriously hinder the formation of the billet shell of the required thickness, and finally cause steel leakage. The loss of the crystallizer taper caused by wear and deformation will lead to a significant increase in the longitudinal cracks at the corners, which is the result of reheating of the corners. In the case of mold deformation, the cause is the thinness of the mold copper plates, which is insufficient to support the thermal expansion of the copper plates. It may also be caused by the damage to the lower part of the mold when the dummy bar is inserted into the mold. Excessive mold taper increases the resistance to billet drawing, resulting in increased mold wear. The reverse taper plus thermal contraction increases the thickness of the air gap, which in turn increases the wear of the corners, so the heat transfer that increases the surface temperature should be reduced. This phenomenon is always accompanied by ferrostatic pressure, which induces tensile strains on the corner surface, which can cause cracks. Such cracks can significantly reduce the shell thickness in a fixed manner and may eventually lead to breakout. The larger the mold corner radius, the larger the air gap. This air gap hinders heat transfer, resulting in a thin shell that is prone to breakout. In slab/bloom continuous casting machines, 4 separate copper plates are fixed to form a cavity surrounding them. If there is an air gap at the joint between 2 copper plates, the primary metal will penetrate into the air gap and begin to solidify, causing hanging at a later stage, resulting in breakout. Therefore, improper adjustment of the crystallizer will affect the heat transfer mechanism and cause steel leakage.
Inappropriate steel level in the copper mould tube
During continuous casting, the steel level in the crystallizer needs to be maintained at 70% to 80% of the crystallizer height. If the steel level drops below the submerged nozzle, the solidified shell formed by the molten steel added later is thin and easy to leak. The steel level drop may occur during the change of the nozzle, the change of the tundish or the blockage of the tundish nozzle. When the flow of molten steel from the tundish into the crystallizer is restricted, if the casting speed is not adjusted, steel leakage may occur. Therefore, if the stopper rod is not properly controlled and causes rotation, the molten steel overflows and sticks to the top of the crystallizer, causing hanging, and the billet drawing is blocked, resulting in steel leakage. The decrease in the steel level will also cause slag inclusion. If there is enough time for the stopper rod to close the submerged nozzle, the steel level can be reduced to below the allowable limit. If casting starts again, the molten steel will suppress the mold protection slag, causing slag inclusion. Therefore, when changing the ladle in full continuous casting, the steel level in the tundish drops. If the operation is not done properly, the tundish slag may enter the molten steel in the crystallizer through the submerged nozzle. The oxidation products of the steel flow, improper deoxidation products, and high-viscosity slag formed by improper aluminum wire spraying in the billet crystallizer, which causes high Al2O3, may penetrate into the billet shell to form slag inclusions, partially inhibit the formation of the billet shell, reduce the lubrication between the billet shell and the crystallizer, and easily stick, resulting in interruption of billet drawing and steel leakage. For the automatic control system of the sizing nozzle, the unstable liquid level of the molten steel in the crystallizer will cause fluctuations in the drawing speed, affect the stable filling of the protective slag between the crystallizer and the billet shell, destroy the continuity of the slag film, and easily make the billet shell thickness uneven, resulting in surface depression or corner cracks and steel leakage. When replacing the submerged nozzle in production, the liquid level fluctuation is relatively large, which is also easy to cause corner cracks or slag rolls and steel leakage.
Eccentricity of the pouring flow of the tundish
The eccentricity of the pouring flow of the tundish leads to uneven heat transfer, resulting in uneven thickness of the solidified billet shell, reduced strength at the weak part of the billet shell, and difficulty in withstanding the static pressure of the molten steel, thus steel leakage. Misalignment of the submerged nozzle, deviation of the center of the steel flow, and severe local scouring of the shell will greatly affect the uniformity of the cooling of the shell in the crystallizer, which may cause steel leakage in severe cases. The intermediate shell is prone to local deformation after a long period of use, resulting in misalignment of the nozzle or inconsistent insertion depth; improper installation of the upper nozzle and the seat brick when repairing the intermediate shell will also cause misalignment of the submerged nozzle. In production, the nozzle alignment can be judged by comparing the depth of the vibration marks on each surface of the leaking steel shell.
Air mist cooling nozzle blockage
The foot roller area is located below the crystallizer, where water is sprayed directly on the shell through the nozzle. The shell is subjected to the pressure of the roller, making it smoother. At this time, the heat transferred is the largest, which is convenient for forming a thicker shell. If the nozzle is blocked, the shell thickness will become thinner, which is easy to cause steel leakage. In case of blockage, it is necessary to apply external force by the pull roller. If it exceeds the limit, it will cause the shell surface to rupture and steel leakage.
Irregularity of dummy bar
Once the molten steel solidifies above the dummy bar of the crystallizer and forms a shell of sufficient thickness, the dummy bar is slowly pulled out. If the dummy bar is not pulled out regularly, steel leakage is likely to occur. Similarly, if the dummy bar is not firmly assembled, the molten steel will flow out of the crystallizer, causing steel leakage. If the dummy bar is separated from the shell prematurely before the dummy bar head is lifted, steel leakage is likely to occur.
Steel leakage Types
Breakouts can be roughly divided into the following categories based on the appearance of the billet shell:
Steel leakage caused by hanging or sticking
The molten steel sticks to the crystallizer, which is called sticking or hanging. This may be caused by improper lubrication between the crystallizer and the billet shell or improper adjustment of the crystallizer, which may be caused by poor quality protective slag, slag inclusions in the billet shell in the crystallizer, overflow of molten steel in the crystallizer, corner seams of the crystallizer, poor/uneven lubrication of the billet continuous caster, etc.
Steel leakage caused by cracks
Both longitudinal cracks at the corners of the billet shell and longitudinal cracks on the wide surface can cause breakouts. If longitudinal cracks cause breakouts, uneven flow of protective slag, uneven heat transfer in the crystallizer leads to uneven billet shell thickness, improper selection of protective slag and uneven cooling of the crystallizer cause the billet shell to break during cooling. For breakouts caused by longitudinal cracks at corners, the billet shell with insufficient solidification thickness along the narrow surface of the crystallizer breaks due to tensile stress during shrinkage. The tensile stress is caused by the reduced taper of the narrow surface of the crystallizer and uneven heat transfer on the narrow surface.
Slag inclusion and steel leakage
The inclusion of protective slag or large inclusions in the billet shell leads to reduced heat transfer, forming a thin billet shell and steel leakage. During the continuous casting of square billets, secondary oxidation products, improper deoxidation products in high-viscosity slag during low-carbon steel smelting, and improper aluminum wire spraying in the crystallizer cause high Al2O3, which all promote slag inclusion in the billet shell, inhibit the growth of the billet shell, and cause steel leakage.
Thin shell steel leakage
Observation of this type of steel leakage in the billet continuous casting machine is caused by uneven billet shell thickness in the crystallizer. The reason may be eccentricity of the pouring flow in the crystallizer or severe deformation of the crystallizer cooling tube.
Steel leakage caused by stopping pouring
If the continuous casting process is interrupted and the pouring is not stopped, the entire furnace of steel will leak out if the connection point cannot withstand the tension applied by re-casting. Measures to control steel leakage Considering the impact of steel leakage on the utilization and effectiveness of the continuous casting machine, necessary measures must be taken to control the occurrence of steel leakage.
- Temperature measurement is only carried out after argon is blown on the casting platform to ensure temperature uniformity. According to the chemical composition of the steel, the pouring stream temperature must be kept superheated by about 60°C before the ladle can be placed on the turntable to ensure that the molten steel is superheated by 25-35°C in the tundish.
- Control the pulling speed according to the temperature monitored in the ladle. The carbon content in the steel is always constant, ensuring that the temperature increases with the decrease of the pulling speed and the pulling speed increases with the decrease of the temperature. Therefore, the pulling speed should be adjusted correctly according to the temperature and carbon content of the steel. Gradually increase the pulling speed to maintain steady-state continuous casting at a certain pulling speed. Any interruption in continuous casting requires a reduction in the pulling speed.
- Any protective slag has an expiration date, so it should not be used after it expires. The mold powder can only be opened during casting and placed under a high-wattage lamp to dry. Do not use the mold powder in an open bag for recasting. Choose the appropriate mold powder according to the specified steel chemistry. At the beginning of casting, use an initial mold powder with low viscosity and low melting point. For billet continuous casting machines, ensure that the linseed oil is evenly distributed in the crystallizer.
- For slab/bloom continuous casting machines, measure the slag pool thickness to determine whether the slag pool thickness exceeds 10mm and the equipment stroke consisting of steel, copper and aluminum wires attached to the steel plate, which helps to avoid slag inclusions and lubricate the billet shell evenly.
- For high-speed billet continuous casting machines, a variety of taper crystallizers can be used instead of traditional linear taper crystallizers. Check the deformation of the crystallizer (if any). Choose the appropriate crystallizer taper and adjust the taper to adapt to the narrow face according to the steel rhyme grade and its solidification method on the slab/bloom continuous casting machine.
- Before the start of continuous casting, check the water flow in the crystallizer 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 rate, as well as the flow equipment. The water quality should also be checked. According to the grade of steel and its solidification mode, adjust the mold cooling mode, that is, the water flow rate (1/min) to adapt to various mold surfaces. To control sticking, use thermocouples to detect the change in the mold wall temperature and reduce the casting speed so that the shell continues to grow evenly. For a given continuous casting machine, ensure that the difference between the inlet and outlet water temperatures does not exceed the specified value during continuous casting.
- Ensure that the maximum radius of the fillet along the copper plate is 0.2mm. If the corner seam exists at the joint of the copper plate, fill the corner seam with gypsum or lime before starting continuous casting.
- Install an automatic mold liquid level controller on the continuous casting machine to maintain the steel liquid level in the crystallizer. To distinguish between molten steel and slag in the crystallizer and check for slag inclusions, install an electromagnetic sensor on the crystallizer.
- Before casting, adjust the tundish water outlet and perform centering. To deal with the blockage of the tundish water inlet, before placing the ladle on the turntable, ensure that the wire of the Ca-Si core is sprayed in, which meets the requirements of high-aluminum steel, so as to form low-melting calcium aluminate. Use a chiller to prevent the plug rod from rotating.
- Ensure proper deoxidation products by using tundish metal protective flux and using shielding plates between the ladle and the tundish to prevent the formation of secondary oxidation products. For the billet continuous casting machine, maintain Mn/Si>3.
- Seal the ingot rod head with asbestos rope, use a chill box, and ensure the correct distribution of the chill box before casting.
- To determine the blockage (if any), check the spray cooling nozzle and water flow.
Analysis of the causes of steel leakage in continuous casting of small square billets
The steel leakage phenomenon of continuous casting of small square billets can be divided into: corner crack steel leakage, middle steel leakage, break steel leakage, and start steel leakage.
The analysis of the causes of steel leakage in continuous casting can be divided into: 1. Steel leakage caused by improper operation. 2. Unreasonable superheat of steel. 3. Steel leakage caused by mold protection slag. 4. The arc of the crystallizer is not good. 5. The vibration frequency and amplitude of the crystallizer are unreasonable. 6. The secondary cooling spray water is unreasonable. 7. The assembly of the crystallizer is unreasonable.
Specific analysis:
1.Causes caused by improper operation
The water inlet of the crystallizer is not centered, resulting in uneven cooling of the temperature of the molten steel in the crystallizer, causing uneven thickness of the billet shell to leak steel.
The steel liquid level is not kept, resulting in too low steel liquid leakage or overflow of steel, resulting in steel billet breakage and leakage. Or due to various production reasons, the rhythm is unstable, resulting in large fluctuations in the pulling speed, the solidification curve deviates from the inner cavity curve of the copper tube, and the thickness of the billet shell is prone to unevenness. In the later period of the use of the crystallization copper tube, it is easy to cause corner cracks and steel leakage at the crystallizer mouth. Corner cracks and steel leakage often occur in a short time after the pulling speed is adjusted. Therefore, the pulling speed should be kept stable as much as possible. The pulling speed cannot be adjusted to adapt to the molten steel temperature, smelting cycle and steel supply rhythm. Instead, the molten steel supply and molten steel quality should be actively guaranteed to meet the continuous casting needs; the life of the submerged nozzle is short and the replacement is frequent. When replacing, the tundish needs to be raised as a whole. The insertion depth of other flow nozzles is too shallow, and the liquid level is unstable, which is easy to cause slag rolling and steel leakage; the original nozzle refractory is not matched, and cold steel is connected between the upper and lower nozzles. The oxide slag formed by blowing and burning with a small oxygen tube enters the crystallizer, which is easy to cause slag leakage.
The slag ring in the crystallizer is not removed in time, resulting in slag rolling and steel leakage of the casting.
The nozzle is blocked or mechanically broken, causing steel leakage.
2.Unreasonable overheating of steel
Crack leakage is closely related to the temperature of the tundish and the drawing speed. Ensuring that the molten steel has a certain degree of overheat can ensure that the molten steel is poured smoothly. Theoretical studies have shown that for every 10°C increase in overheating, the thickness of the billet shell at the outlet of the crystallizer decreases by 3%. If the temperature is too high, the billet shell at the outlet of the crystallizer will be thin and the high-temperature strength will be low. Once the stress is torn through the billet shell, crack leakage is likely to occur.
3.Steel leakage caused by crystallizer protective slag
1) The protective slag is not added in time, resulting in no lubrication between the ingot and the copper tube of the crystallizer.
2) The selection of protective slag is unreasonable, that is, the melting point and dissolution rate are unreasonable. The role of the crystallizer protective slag: insulation, prevention of secondary oxidation, absorption of inclusions, lubrication of the billet shell and the copper tube of the crystallizer, and reduction of friction resistance. There are many varieties of continuous casting crystallizer protective slag.
(1) According to the chemical composition of the base metal, it can be divided into: SiO2-Al2O3-CaO system, SiO2-Al2O3-FeO, SiO2-Al2O3, Na2O system, among which the former is the most commonly used, on which a small amount of additives (alkali metal or alkali metal oxide, fluoride, boride, etc.) and carbonaceous materials (carbon black, graphite and coke, etc.) to control the melting rate are added.
(2) According to the shape of the protective slag, it can be divided into powdered slag (mechanical mixing molding), granular slag (extruded products are long strips, disc-formed products are round, and spray-formed products 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 protective slag (low, medium and high carbon steel protective slag and special steel special slag), exothermic type pouring slag, etc. Selection principles for continuous casting mold protection slag: The continuous casting mold protection slag should have reasonable melting temperature, melting rate and molten layer structure in the crystallizer; stable and appropriate viscosity; sufficient ability to absorb inclusions in the steel.
4.The arc alignment is not good, which causes the crystallizer movement to have shear force on the ingot and steel leakage.
5.Crystallizer vibration frequency and amplitude.
The crystallizer vibration frequency and amplitude are not selected reasonably. The crystallizer has no negative slip or small negative slip, which causes steel leakage.
The vibration arc simulation is poor and the deflection is large, which will produce shear force on the ingot shell, affect the lubrication of the protective slag, and increase the resistance of the billet drawing. From the perspective of heat transfer, the vibration arc simulation is poor and the deflection is large, which will increase the unevenness of the air gap between the ingot shell and the copper tube, resulting in an increase in the difference in the thickness of the ingot shell. The general reason is that the vibration is unstable due to the steel slag stuck on the east side of the vibration frame or the leaf spring is damaged, the crystallizer deflects seriously, the ingot shell wears seriously on both sides of the inner cavity of the copper tube and the inner east corner, and the uneven heat transfer at the deflection corner causes corner cracks and steel leakage.
6.Secondary cooling spray water.
The ingot shell temperature is high and loses support just after the ingot leaves the crystallizer. At this time, uniform strong cooling is required to promote the rapid growth of the ingot shell. If the local cooling of the upper part of the secondary cooling is too weak, remelting will occur, causing steel leakage.
7. Crystallizer
1) When assembling the crystallizer, the uneven water joints between the water jacket and the copper tube cause uneven cooling of the copper tube, and the heat conduction is low on the side with low flow, resulting in thin shell and steel leakage of the ingot.
2) The taper of the copper tube of the crystallizer is unreasonable. The thermal resistance of heat transfer in the crystallizer is mainly the air gap. If the air gap is small, the thermal resistance is small, and if the air gap is large, the thermal resistance is large. In the early stage of the crystallizer, the inner cavity curve of the copper tube is close to the contraction curve of the shell, the air gap is uniform, the heat transfer is uniform, and the thickness of the shell is also uniform. During use, the copper tube 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 curved liquid surface is large, the copper tube is partially deformed, and the unevenness of the shell is also increased. The shell is prone to bulging at the bottom of the crystallizer. Sampling shows that the bulging amount of 150mm×150mm ingot is greater than 2mm, which is prone to deviation angle cracking. After leaving the crystallizer, the shell loses support and is prone to steel leakage. Influence on the inner surface of the copper tube: During the use of the copper tube, due to excessive and crooked cold steel placed in the treatment of steel leakage accidents, the face and corners are scratched, with a depth of more than 1mm. During the steel drawing process, the thermal resistance between the blank shell and the copper tube wall at the scratch is large, the blank shell is thin, and it is easy to be depressed, and there are obvious cracks at the bottom of the depression. At this time, if the overheating increases or the pulling speed is suddenly increased, it is easy to leak steel at the crack. The copper tube is poorly made, especially the partial shedding of the coating on the face and corners of the copper tube. The thermal resistance is increased, resulting in unstable heat transfer, which is easy to cause steel leakage. In addition, there are sand holes in the copper tube blank. As the amount of steel passing through the crystallizer increases, the inner surface of the copper tube wears, the sand holes leak out, and the phenomenon of hanging steel occurs. In severe cases, cracks cause steel leakage.
