Longitudinal cracks on the surface of continuous casting billets: a complete analysis of causes, hazards and prevention
In the continuous casting process of steel production, longitudinal cracks on the surface of continuous casting billets are a problem that cannot be ignored. It not only affects the quality of the billet, but also causes major production accidents such as steel leakage in severe cases. Today, let us take a deeper look at the longitudinal cracks on the surface of continuous casting billets.
Definition and appearance characteristics
Longitudinal cracks on the surface of continuous casting billets refer to cracks generated near the center of the billet surface along the billet drawing direction. These cracks are between 10-1500mm in length, 0.1-3.5mm in width, and less than 5mm in depth. On the surface of the billet, they are like scars, which directly affect the appearance quality of the billet.
Cause analysis
1.Uneven thickness of the solidified shell in the meniscus area of the crystallizer: In the meniscus area of the crystallizer (about 170mm below the steel liquid surface), a δ→γ transformation will occur during the solidification of the steel liquid. This transformation occurs below the solidus line, which will lead to uneven solidification thickness. Due to thermal shrinkage, the shell will produce stress gradients, and stress will concentrate at weak points, so that the shell will form longitudinal depressions on the surface, and finally form longitudinal cracks. This is the root cause of surface longitudinal cracks.
2.Influence of molten steel composition
When [S]＞0.020% and [P]＞0.017% in molten steel, the probability of surface longitudinal cracks will increase.
Reducing Mn/S is not conducive to reducing cracks, and a suitable Mn/S ratio helps to improve the crack resistance of steel.
When [C]=0.12 – 0.17%, it belongs to peritectic reaction steel. During the δ→γ transformation process, the shrinkage is large, air gaps will be formed, resulting in shell wrinkles, unstable heat flow in the crystallizer, and further aggravating the unevenness of shell thickness.
3.Pulling speed factor: Increasing the pulling speed will shorten the cooling time of the ingot in the crystallizer, and the shell growth will be insufficient, thereby increasing the risk of surface longitudinal cracks.
4.Protective slag problem
Poor melting performance of the protective slag, too thick or too thin liquid slag layer will lead to uneven slag film thickness. When the thickness of the liquid slag layer is less than 10mm, the slag film cannot evenly protect the ingot, which will make the local solidified shell too thin and prone to cracks.
The ideal protective slag should meet η•ν=2 – 4, the thickness of the liquid slag layer is 10 – 15mm, select protective slag with high crystallization temperature, and ensure uniform slag film thickness (150μm/0.3 – 0.5kg/m2).
5.Crystallizer liquid level fluctuation: When the crystallizer liquid level fluctuation is ≥5mm, it will interfere with the normal solidification process of the ingot, make the shell growth uneven, and increase the possibility of cracks. It is more appropriate to ensure that the crystallizer liquid level fluctuation is ±3 – ±5mm.
6.Crystallizer heat flow and cooling • For low carbon steel, when the crystallizer heat flow is greater than 60Cal/cm²; for medium carbon steel, when the crystallizer heat flow is greater than 41Cal/cm², the billet cools too fast or unevenly, which will cause the generation of surface longitudinal cracks. • To ensure the uniform growth of the initial billet shell in the crystallizer, appropriate crystallizer taper, weak crystallizer cooling, hot top crystallizer and other measures can be adopted.
7.Crystallizer steel liquid flow
Misalignment of the water nozzle will cause uneven flow of molten steel in the crystallizer, impact the billet shell, and affect the uniform growth of the billet shell.
Improper water nozzle insertion depth will also change the flow state of molten steel, which is not conducive to the solidification of the billet and increases the risk of cracks.
8.Crystallizer vibration
The depth of vibration marks will affect the surface quality of the billet and increase the probability of cracks.
The increase of negative slip time will cause the billet to be subjected to additional stress during solidification, which will promote the formation of cracks. The appropriate negative slip value, frequency and amplitude should be controlled, and the vibration deviation should be ensured (longitudinal and lateral <0.2mm).
Hazards
When the surface longitudinal cracks are serious, it will cause steel leakage accidents, which will not only lead to production interruptions, but also damage equipment and increase production costs. At the same time, steel billets with serious cracks cannot meet the requirements of subsequent processing and can only be judged as scrap, resulting in a waste of resources.
Prevention and elimination methods
1.Control the composition of molten steel: try to reduce the content of [S] and [P] in steel, increase Mn/S, and reduce the possibility of cracks from the source.
2.Optimize the billet drawing speed: select a suitable billet drawing speed to ensure that the billet has enough cooling time in the crystallizer to allow the billet shell to grow fully.
3.Select suitable protective slag: protective slag that meets the conditions of η•ν=2 – 4, liquid slag layer thickness of 10 – 15mm, high crystallization temperature, uniform slag film thickness (150μm/0.3 – 0.5kg/m2) can effectively protect the billet and reduce cracks.
4.Ensure the rationality of molten steel flow: control the crystallizer liquid level fluctuation within the appropriate range, ensure the nozzle centering and the appropriate nozzle insertion depth, so that the molten steel flows evenly in the crystallizer.
5.Promote uniform growth of the billet shell: adopt appropriate crystallizer taper, weak cooling of the crystallizer, hot top crystallizer and other methods to ensure uniform growth of the initial billet shell of the crystallizer.
6.Reasonably control the vibration of the crystallizer: set the appropriate negative slip value, frequency and amplitude, control the vibration deviation, and reduce the risk of cracks caused by vibration.
7.Ensure the good condition of the equipment: ensure that the crystallizer is accurately aligned with the zero section and the upper part of the second cooling zone, and the cooling uniformity is good, so that the billet out of the crystallizer can operate normally.
Inspection and disposal measures
1.Inspection method: Check the surface of the billet with the naked eye to find the surface longitudinal cracks in time.
2.Disposal method: For the cracks found, flame cleaning is carried out. If the defect is serious, the part is directly cut off and scrapped to prevent unqualified products from entering the subsequent production links.
The control of longitudinal cracks on the surface of continuous casting billets is a systematic project, which needs to start from multiple aspects such as molten steel composition, production process, equipment operation, etc. Only by comprehensively controlling each link can we effectively reduce the generation of longitudinal cracks on the surface, improve the quality of the billet, and ensure the smooth progress of steel production. I hope that today’s content can give you a deeper understanding of longitudinal cracks on the surface of continuous casting billets.
