In the steel production process, the quality of continuous casting billets is of vital importance, and longitudinal cracks in the corners are one of the most common and harmful defects in continuous casting billets. Today, let’s take a deep look at the causes and control methods of longitudinal cracks in the corners of continuous casting billets.
1. Hazards of longitudinal cracks in corners
Longitudinal cracks at the corners of continuous casting billets usually appear at 0-15mm from the corners (edges) of the billets, 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 serious cases, they can cause steel leakage accidents, causing the billets to become waste directly, greatly affecting the quality and output of steel production and increasing production costs. Usually, waste caused by longitudinal cracks at 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.
2. In-depth analysis of the causes
(I) Influence of molten steel composition
6. 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, this problem occurred when the original design water flow was 120 – 130m³/h.
If the continuous casting speed is too arbitrary, it is not good to be too fast or too slow. If the 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 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.
1. Crystallizer liquid level fluctuation: Crystallizer liquid level fluctuation has a significant impact on the generation of longitudinal cracks. Tests at the Solmer factory 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 aggravates the crystallizer liquid level fluctuations.2. Nozzle alignment: After long-term use of the tundish, the bottom of the cladding shell protrudes downward severely, making it difficult for the nozzle seat brick to seat properly, and the nozzle alignment accuracy deviation is large. The misaligned nozzle steel flow will wash away the adjacent primary shell, which is easy to cause longitudinal cracks at the weak part of the shell.3. Crystallizer lubrication: If the crystallizer lubrication is unstable, such as when the original automatic lubrication system is changed to artificial rapeseed oil lubrication, the control fluctuations of refueling time, quantity, frequency, etc. will be large, resulting in poor continuity and uniformity of lubrication, which will cause uneven growth of the billet shell and produce longitudinal cracks at the corners.3. Control strategy
2. Optimize the inverted taper curve: Change the taper of the copper tube from a single taper to a parabolic taper, so that it is closer to the billet shrinkage curve, reduce the gap between the billet and the copper tube, and improve the heat transfer effect.
3. Reasonably set the fillet radius: Change the fillet radius of the 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.
4. Accurately adjust the water gap: When repairing the crystallizer, use a feeler gauge to check the water gaps on all four sides, accurately adjust the width of the water gap, ensure that the water gaps on all sides are uniform, and enable the billet shell to cool and grow evenly in the crystallizer.
2. Ensure nozzle alignment: Regularly inspect and maintain the tundish, replace the deformed tundish bottom steel shell in time, ensure the sizing nozzle is aligned (deviation ≤±2mm), and avoid the nozzle steel flow scouring the primary shell.
3. Optimize crystallizer lubrication: Ensure the continuity and uniformity of crystallizer lubrication. Use stable lubrication methods and lubricants to avoid uneven shell growth due to lubrication problems.
