Slab sticking and leakage is the most common (50%-70%) serious accident in slab continuous casting. Its various causes have been detailed above. So how do we customize preventive measures?
We need to focus on various aspects, including steel quality, process stability, high lubrication performance, equipment assurance, and early warning systems. The main aspects are as follows:
Stable Process Control
Stable control of process parameters is the core triggering factor for sticking and leakage. In particular, matching different casting speeds and cooling regimes according to different slab sizes ensures that the slab shell thickness is uniform and its strength is sufficient to resist the static pressure of casting and molten steel, thus preventing leakage.
(1) Matching different casting speeds to slab sizes: The principle of “the wider the slab, the lower the casting speed; the thicker the slab, the stricter the casting speed; and dynamic adjustment to adapt to the cross-section” should be followed. For example, for slabs with different widths such as 1000mm, 1350mm, 1500mm, and 1550mm, the upper limit should be set according to the principle of “the wider the slab, the lower the casting speed.” The thickness of the billet shell exiting the crystallizer must be ≥10mm (for thin billets) or 15mm (for thick billets); otherwise, it cannot withstand the pulling force (usually ≥12kN) and the static pressure of molten steel (up to 0.3MPa at the center of the wide face). The larger the billet size, the more significant the heat flow fluctuation (e.g., the heat flow fluctuation of a 1550mm wide slab is 6 times that of a 1350mm slab), requiring a low pulling speed to reduce heat loss imbalance and resulting in uneven billet shell.
(2) Stable control of pulling speed: Fluctuations in pulling speed can disrupt slag lubrication and cause uneven billet shell. A sudden increase in casting speed can cause the slag inflow rate to become mismatched with the casting speed. For example, from 0.8 m/min to 1.0 m/min, the slag layer thickness decreases by 0.5-1 mm per second, dropping from 12 mm to below 5 mm within 3-5 seconds, forming a “slag-free zone.” Conversely, a sudden decrease in casting speed prolongs the residence time of molten steel in the crystallizer, increases the amount of protective slag melted, and causes the slag layer thickness to increase sharply to 18 mm. Some slag forms a “thick slag ring (>3 mm)” on the meniscus, blocking the slag inflow channel. When the casting speed recovers, the slag cannot be replenished in time, still causing adhesion. Furthermore, a sudden increase in casting speed shortens the solidification time, reducing the thickness of the billet shell exiting the crystallizer. An excessively thin shell thickness in the center of the wide face will not be able to withstand the hydrostatic pressure of the molten steel (0.3 MPa), forming a localized “thin shell zone.”
Ensuring the Lubricating Ability of the Protective Slag
The melting point and viscosity of the protective slag need to be adjusted according to the characteristics of the steel. Low-carbon aluminum killed steel requires a low-viscosity (0.08-0.12 Pa・s at 1300℃) and low-melting-point (1050-1080℃) protective slag to avoid viscosity increases caused by Al₂O₃ inclusions. Medium and high-carbon steels (such as No. 45 steel) require an increased glass phase ratio (60%-70%) and a decreased crystalline phase ratio (<30%) to reduce slag ring formation, etc. Additionally, Li₂O, Na₂O, and other components need to be added to the protective slag to improve its resistance to Al₂O₃ inclusions.
The lubricating effect of the protective slag is also reflected in its application: protective slag consumption per ton of steel ≥ 0.4 kg, and the liquid slag layer thickness maintained at 10-15 mm. When the liquid level fluctuation in the crystallizer exceeds ±5mm, a thick slag ring (>3mm) is easily formed. This needs to be gently removed with a special tool during the liquid level drop to avoid damaging the liquid-slag film. Slag rings are more likely to form in the middle of the wide face; these should be checked every 30 minutes, and if the slag ring thickness exceeds 2mm, it should be addressed promptly. Maintain a powdered slag layer thickness of 80-100mm to prevent excessive heat loss and insufficient melting of the liquid slag due to excessively thin powdered slag. If the powdered slag layer thickness is <80mm, protective slag needs to be added to prevent the liquid-slag layer thickness from dropping below 8mm.
High-Precision Equipment Maintenance
High-precision maintenance of the crystallizer is crucial to preventing steel adhesion and leakage. Before the crystallizer goes online, the inverted taper should be set according to the shrinkage characteristics of the steel grade: 3.0-3.5mm for the narrow face and 0.9%-1.4% for the wide face. Excessive taper (>3.5mm) increases friction between the billet shell and the copper plate (by 30%), while insufficient taper (<2.5mm) increases the air gap and leads to inadequate slag filling. Calibration with a laser taper meter is necessary after every few tons of steel cast, with adjustments made when the deviation exceeds ±0.1mm. After each casting cycle, the copper plate in the crystallizer must be inspected and confirmed. Scratches deeper than 0.5mm require grinding and repair; plates with peeling plating need replacement; and plates with severe wear at the bottom edge or improper taper require removal from the production line and replacement.
Preventing and controlling slag adhesion requires proactive measures, primarily through ensuring lubrication with protective slag, maintaining stable process parameters, and precise equipment maintenance. Ultimately, the slag adhesion rate should be controlled below 1%, ensuring stable and smooth continuous casting production.
FAQ
How does improper taper setting in the copper mould tube directly lead to sticking or steel leakage?
A: The function of the taper is to compensate for the gaps created by billet shrinkage.
Too small a taper (<2.5mm): This results in an excessively large air gap between the billet shell and the copper wall, increasing thermal resistance, slowing down billet cooling, and making the billet shell thinner and more susceptible to breakage or puncture by the hydrostatic pressure of the molten steel.
Too large a taper (>3.5mm): The billet shell is tightly pressed against the copper plate, increasing friction by approximately 30%. During vibration, excessive friction can easily tear the fragile nascent billet shell, directly inducing sticking.
Recommendation: The narrow side is typically set to 3.0-3.5mm, and the wide side to a ratio of 0.9%-1.4%, and calibrated regularly using a laser measuring instrument.
How does the steel leakage prediction system use thermocouples to identify “sticky steel leakage” signals?
A: The system monitors temperature trends through multiple layers of thermocouples embedded in the copper plate.
V-shaped wave characteristics: When adhesion occurs, the molten steel at the tear directly contacts the copper plate, causing a sudden rise in the thermocouple temperature at that point. As the casting speed decreases, this high-temperature point will sequentially conduct to the thermocouples below.
Identification logic: If the system detects that the temperature peak moves in a “top layer rises first, bottom layer rises later” pattern on the time axis, it is determined to be a “V-shaped adhesion breakage”. At this time, the system will automatically trigger a deceleration program, allowing the breakage time to heal within the COPPER MOULD TUBE.
What are the significant impacts of cooper mould tube cooling water quality on preventing steel leakage?
A: Cooling water quality directly determines the heat exchange efficiency of the cooper mould tube.
Scale formation risk: If the cooling water has excessive hardness or contains impurities, scale will form on the outer wall of the copper tubes (at the water joints). Scale has extremely high thermal resistance, leading to localized overheating of the copper plate.
Deformation and wear: Localized overheating will reduce the hardness of the copper plate, accelerate wear, and even cause permanent thermal deformation. A deformed cooper mould tube will result in extremely uneven cooling of the billet shell, forming longitudinal cracks, which can eventually lead to steel leakage. Therefore, the water system filter screen must be cleaned regularly, and the temperature difference between the inlet and outlet water must be strictly monitored (it should not exceed the specified value).
