In the process of steel production, the quality of continuously cast square billets is of vital importance, and longitudinal cracks in corners are one of the common and most harmful defects of continuously cast square billets. Today, let’s take a deep look at the causes and control methods of longitudinal cracks in corners of continuously cast square billets.
Harm of longitudinal cracks in corners
Longitudinal cracks in corners of continuously cast square billets usually appear at 0-15mm from the corner (edge) of the billet, with a crack length of 10-1500mm, a width of 0.1-3.5mm, and a depth of less than 5mm. Don’t underestimate these cracks. In severe cases, they will cause steel leakage accidents, causing the billet to become waste directly, greatly affecting the quality and output of steel production and increasing production costs. Usually, waste caused by longitudinal cracks in corners accounts for 75% of the total, and accident stoppages caused by corner cracks account for 60% of the total stoppages. The metal recovery rate is reduced, the consumption of steel materials is increased, and the operation rate of the continuous casting machine is also limited.
In-depth analysis of the causes
(I) Influence of molten steel composition
- Sulfur content: When the sulfur content in molten steel is high, low-melting eutectic compounds will be formed on the grain boundaries, such as FeO-FeS, whose melting point is only 940 °C, which will greatly reduce the strength of the shell and make longitudinal cracks more likely to occur. Studies have shown that when the sulfur content is greater than 0.025%, the surface cracks increase significantly. Moreover, the effect of sulfur on cracks is closely related to the pouring temperature. When pouring at high temperatures, sulfur is more harmful. For example, in Anshan Iron and Steel’s research, when the tundish temperature is higher than 1540°C and the sulfur content in the molten steel is high, two-thirds of the molten steel has corner cracks and steel leakage.
- Manganese-sulfur ratio: Increasing the manganese-sulfur ratio can generate FeS-MnS eutectic compounds with a higher melting point, thereby reducing the harm of sulfur. Generally speaking, when Mn/S is greater than 20, cracks will be greatly reduced. For A3 steel, it is important to maintain Mn/S not less than 15.
- Carbon content: When producing steel with carbon content within the peritectic reaction range, the peritectic reaction will be accompanied by a large volume shrinkage, increasing the air gap between the primary shell and the copper tube wall, affecting the heat flow distribution of the shell, making the shell cooling uneven, and making it easier for weak points to appear at the corners, thereby forming cracks. However, in different studies, there is some controversy about the effect of carbon content on longitudinal cracks at corners. Some studies believe that within the range of 0.10% – 0.18%, an increase in carbon content will increase crack sensitivity; while within the range of 0.14% – 0.22%, some studies have not found that an increase in carbon content has an adverse effect on the crack tendency of A3 steel.
- Phosphorus content: Phosphorus, like sulfur, also reduces the high-temperature plasticity of steel, and its segregation and inclusions will provide crack sources. When the phosphorus content is less than 0.017%, the effect on the crack sensitivity of steel is small, but as A3 steel, the phosphorus content is usually controlled in a low range.
(II) Influence of molten steel temperature
Many steel mills face the problem of molten steel temperature control in production. Due to the unstable composition of molten iron and the mixed types of scrap steel, the tapping temperature is difficult to control. In addition, the ladle and tundish have small contents, the baking state is different, and the process temperature drop is difficult to control. High superheat casting often occurs. At the same drawing speed, high superheat will make the billet shell thinner, form greater stress concentration at the corners, and thus produce longitudinal cracks at the corners. In severe cases, it may even cause longitudinal cracks at the corners and steel leakage accidents.
(III) Crystallizer-related factors
- Copper tube length: If the original copper tube length is short, such as the original copper tube length of a factory is 850mm, the cooling time of the ingot will be insufficient, and the thickness of the ingot shell cannot be formed before leaving the crystallizer, which increases the risk of longitudinal cracks at the corners.
- Inverted taper curve: If the inverted taper curve of the copper tube deviates greatly from the solidification curve of the steel billet, like the copper tube with the original single taper curve, the gap between the ingot and the copper tube is large, the heat transfer effect is poor, which is not conducive to the uniform growth of the billet shell and is easy to cause longitudinal cracks at the corners.
- Fillet radius: If the fillet radius of the copper tube is too large, the air gap at the corner will be larger and the longitudinal crack index will be higher. For example, when the fillet radius is 10mm, there is such a problem; and after changing it to 8mm, the cooling conditions at the corners can be improved to a certain extent and the longitudinal cracks can be reduced.
- Water seam: When repairing the crystallizer, if the water seams around are adjusted based on experience, the error is large, resulting in uneven width of the water seams on all sides, which will cause poor cooling of the corners of the crystallizer and cause longitudinal cracks in the corners.
- Lubrication and heat transfer conditions: If open protective slag is used for casting and the amount of protective slag added is small, the molten steel surface of the crystallizer will not form a slag layer, the liquid slag will penetrate into the gap between the crystallizer and the billet less, the heat transfer effect will be poor, and the uniform growth of the billet shell will be affected. In addition, if there are scratches or poor lubrication in the corner area of the copper tube wall, the billet shell will be in direct contact with the copper tube wall, and the heat transfer will be too fast, which will cause the billet shell to shrink prematurely and form a depression. As the billet is drawn, the thermal resistance at the depression increases, the billet shell becomes thinner, and stress concentration points appear, resulting in longitudinal cracks under the action of thermal stress.
- Water flow rate: If the water flow rate of the crystallizer is unreasonable and the flow rate is too fast, the temperature difference between the inlet and outlet water of the crystallizer will be too low, and a turbulent area will be generated, which will affect the growth of the primary billet shell, resulting in local weakness and then longitudinal cracks. For example, when the original design water flow rate was 120-130m³/h, such a problem occurred.
(IV) The influence of secondary cooling water distribution
If the secondary cooling water distribution adopts manual experience water distribution and each flow cannot be controlled separately, it is difficult to reasonably adjust the water volume according to the temperature change of the billet. This will lead to a large temperature gradient and generate large thermal stress, which will not only easily form new cracks, but also further expand the existing cracks.
(V) The influence of drawing speed
If the continuous casting drawing speed is more arbitrary, it will not work if it is too fast or too slow. If the drawing speed is too fast, the cooling time of the billet is short and uneven, and the billet shell is thin and more uneven when it comes out of the crystallizer, which will promote the generation and development of cracks; if the drawing speed is too slow, the cooling intensity of the billet surface is too large, the temperature gradient increases, and the thermal stress increases, which is also not conducive to preventing longitudinal cracks at the corners.
(VI) The influence of steel pouring operation
- Crystallizer liquid level fluctuation: Crystallizer liquid level fluctuation has a significant impact on the generation of longitudinal cracks. Tests at the Solmer plant in France show that when the liquid level fluctuation is greater than 10mm, the probability of longitudinal cracks is 30%; when the liquid level fluctuation increases from ±5mm to ±20mm, the longitudinal crack index increases from 0 to 2.0. Fluctuations in production factors and temperature fluctuations between furnaces may cause fluctuations in the tundish liquid level, which in turn exacerbates the crystallizer liquid level fluctuations.
- Sprue alignment: After long-term use of the tundish, the bottom of the cladding shell bulges downward severely, making it difficult for the sprue seat brick to seat properly, and the sprue alignment accuracy deviation is large. The misaligned sprue steel flow will wash the adjacent primary shell, and it is easy to cause longitudinal cracks at the weak part of the shell.
- Sprue lubrication: If the crystallizer lubrication is unstable, such as after the original automatic lubrication system is changed to artificial rapeseed oil lubrication, the control fluctuations of refueling time, quantity, frequency, etc. are large, resulting in poor continuity and uniformity of lubrication, which will cause uneven growth of the shell and cause longitudinal cracks at the corners.
Control strategy
(I) Optimize molten steel composition
- Control sulfur content: During the production process, the sulfur content in molten steel must be strictly controlled. For different steel grades, formulate corresponding sulfur content standards. For example, when producing Q235 series, S is controlled at the middle and lower limits; when producing HRB335 and HRB400, S is required to be ≤0.035%.
- Adjust manganese-sulfur ratio: Improving the manganese-sulfur ratio is an important measure to reduce longitudinal cracks at corners. By reasonably adjusting the manganese and sulfur content, ensure that Mn/S reaches a certain standard. For example, for A3 steel, try to maintain Mn/S not less than 15.
- Stabilize carbon content: For steel grades with carbon content within the range of peritectic reaction, strictly control the temperature and composition of molten steel to reduce the adverse effects of peritectic reaction.
(II) Accurately control molten steel temperature
- Set up a nitrogen blowing station: Set up a nitrogen blowing station on the continuous casting platform. When encountering high-temperature steel, add clean scrap steel to adjust the temperature, and strengthen nitrogen blowing and stirring to improve the uniformity of molten steel composition and temperature.
- Strengthen baking and heat preservation: Strengthen the baking and heat preservation process of ladle and tundish, reduce the temperature drop during the process, make the tundish tend to be poured at a constant temperature, control the superheat at 15-30℃, and avoid the billet shell being too thin due to high superheat.
(III) Improve crystallizer parameters
- Adjust the length of copper tube: appropriately increase the length of copper tube, such as changing the length of copper tube from 850mm to 900mm, to provide more sufficient cooling time for the billet, so that the billet shell can reach sufficient thickness before leaving the crystallizer.
- Optimize the inverted taper curve: change the taper of copper tube from single taper to parabolic taper, so that it is closer to the shrinkage curve of billet, reduce the gap between billet and copper tube, and improve the heat transfer effect.
- Reasonably set the fillet radius: change the fillet radius of copper tube from a larger value (such as 10mm) to a suitable value (such as 8mm), reduce the corner air gap, and improve the corner cooling conditions.
- Accurately adjust the water gap: When repairing the crystallizer, use a feeler gauge to check the water gaps on all sides, accurately adjust the width of the water gaps, ensure that the water gaps on all sides are uniform, and allow the shell to cool and grow evenly in the crystallizer.
(IV) Improve crystallizer lubrication and heat transfer
- Improve the pouring method: Change the original open pouring to the pouring method of the intermediate tank fast water inlet + immersion water inlet + protective slag, so that the protective slag in the molten steel of the crystallizer is clearly layered, and the good filling air gap effect of the liquid slag layer is used to improve the lubrication effect and heat flow distribution, promote uniform thickening of the shell, reduce friction, and avoid the generation of longitudinal cracks.
- Ensure the quality of copper tubes: Strengthen the inspection of copper tubes, promptly find and replace copper tubes with scratches on the corners, and prevent abnormal heat transfer and stress concentration caused by scratches.
(V) Reasonable configuration of crystallizer water flow
Control the crystallizer water pressure at 0.8-0.9MPa, the water flow at 100-110m³/h, and the flow rate at 11-12m/s. This can not only avoid the cooling water running straight in the inner cavity of the crystallizer due to high water pressure, affecting the cooling effect, but also prevent the turbulent flow area caused by too high water flow rate, so as to achieve better cooling effect.
(VI) Optimize the secondary cooling system
The secondary cooling system adopts automatic water distribution mode, and the computer automatically adjusts the flow rate and vibration efficiency according to the drawing speed, and the three flows and three cooling sections can be controlled separately, making the secondary cooling system more reasonable and effectively inhibiting the further deterioration of the longitudinal cracks in the corners.
(VII) Strictly control the drawing speed
Strictly limit the drawing speed, generally requiring less than 3m/min. At the same time, the sizing nozzle should be replaced in time after the diameter is expanded to ensure the stability of the drawing speed, ensure the uniform cooling of the billet, and reduce the generation of cracks.
(VIII) Standardize steel pouring operation
- Stabilize liquid level fluctuation: Strengthen liquid level control, eliminate large fluctuations in the steel level of the tundish through standardized operation and strengthened assessment, and ensure that the fluctuation of the steel level of the crystallizer is ≤±5mm.
- Ensure the centering of the nozzle: Regularly inspect and maintain the tundish, replace the deformed bottom steel shell of the tundish in time, ensure the centering of the sizing nozzle (deviation ≤±2mm), and avoid the nozzle steel flow from scouring the primary billet shell.
- Optimize crystallizer lubrication: Ensure the continuity and uniformity of crystallizer lubrication, and use lubrication methods and lubricants with stable performance to avoid uneven growth of the billet shell due to lubrication problems.
Through in-depth analysis of the causes of longitudinal cracks in the corners of continuous casting billets and taking corresponding effective control measures, steel companies can significantly reduce the probability of longitudinal cracks in the corners, improve the quality of continuous casting billets, improve production efficiency, and reduce production costs. I hope today’s sharing will give you a more comprehensive understanding of the longitudinal cracks in the corners of continuous casting billets, so that you can better deal with this problem in actual production.
