In continuous casting, the crystallizer is a key piece of equipment for the solidification and forming of molten steel, and crystallizer surface protection is a crucial step in ensuring the smooth operation of the continuous casting process and improving the quality of the cast billet. Good surface protection effectively isolates the molten steel from air, prevents secondary oxidation, reduces the formation of inclusions, and stabilizes the solidification process, thus having a profound impact on the internal and surface quality of the cast billet. The following will detail commonly used methods for crystallizer surface protection.
Application of protective slag
Mechanism of action of protective slag
The protective slag plays a crucial role in protecting the molten steel surface in the crystallizer. When the protective slag is added to the surface of the molten steel in the crystallizer, it undergoes a series of complex physicochemical changes. First, the protective slag near the molten steel surface rapidly absorbs the heat provided by the high-temperature molten steel, forming a liquid slag layer on the surface of the molten steel. Above the liquid slag layer is a sintered layer (also known as a transition layer), and on top of that is a loose powdery layer, thus forming a typical three-layer structure.
(1) Isolate from air to prevent secondary oxidation:
The slag layer formed after the protective flux melts evenly covers the surface of the molten steel, acting like a “protective suit” to isolate the molten steel from the air and prevent oxygen, nitrogen, and other gases from contacting it, thus avoiding secondary oxidation. This effect is particularly crucial for steels containing high levels of easily oxidized elements, such as aluminum-killed steel and stainless steel. Secondary oxidation of the molten steel leads to the formation of numerous oxide inclusions, such as aluminum oxide (Al₂O₃). These inclusions severely affect the steel’s properties, reducing its strength, toughness, and fatigue life, and may also create defects on the steel surface, lowering the product’s yield.
(2) Thermal insulation:
The three-layer structure of the protective slag reduces radiative heat loss from molten steel and lowers its superheat. In protective slag operation, a certain thickness of powdery slag layer is typically required above the liquid slag layer, a process known as “black slag operation.” This is because the presence of the powdery slag layer further enhances the heat preservation effect, increases the meniscus temperature of the crystallizer, and reduces the formation or excessive growth of slag rings. Especially when casting high-carbon steel, good thermal insulation properties help improve the lubrication conditions of the billet and prevent defects such as cracks on the billet surface. The heat preservation properties of the protective slag can be improved by increasing the carbon content, changing the type of carbonaceous material, adding exothermic elements, or reducing the bulk density of the protective slag.
(3) Absorbing inclusions in molten steel:
To prevent inclusions floating in molten steel from being drawn into the solidified shell and causing surface or subsurface defects in the cast billet, the slag layer formed by the melting of the protective slag should have the ability to absorb and assimilate non-metallic inclusions floating in the molten steel. When inclusions in the molten steel float to the slag layer, the protective slag can capture and dissolve them, thereby purifying the molten steel and improving the purity of the cast billet. For example, in the production of high-quality automotive panel steel, the effective absorption of inclusions by the protective slag can significantly reduce surface defects in the cast billet and improve the surface quality of the steel.
(4) The lubricating and heat transfer-improving effects of the slag film:
The slag film formed between the melted protective slag flowing into the mold wall and the solidified billet shell acts as a lubricant, reducing casting resistance and preventing the billet shell from sticking to the mold wall. Simultaneously, a suitable slag film thickness and uniformity improve heat transfer uniformity within the mold. The slag film reduces the heat transfer rate at the top and increases the heat transfer rate at the bottom, resulting in a more rational temperature distribution during solidification of the billet, which is beneficial for homogenizing the solidification structure and improving billet quality. Generally, the slag film thickness is typically between 0.1 and 1.5 mm. To ensure good slag film performance, it is necessary to control the melting rate of the protective slag to maintain a 6-15 mm thick liquid slag layer on the surface of the molten steel in the mold.
Selection and maintenance of protective slag
(1) Select the appropriate protective slag according to the steel grade:
Different steel grades have varying performance requirements for fluxing agents. For low-carbon steel, the fluxing agent needs low basicity and good lubrication to ensure the surface quality of the cast billet; while for high-carbon steel, it needs high thermal insulation properties and suitable melting characteristics. When producing special steel grades, such as those containing microalloying elements like titanium and niobium, the fluxing agent also needs to possess corresponding chemical stability to avoid adverse reactions with these alloying elements. Therefore, steel companies need to select specialized fluxing agents based on the characteristics of the steel grades they produce to meet the continuous casting process requirements of different steel grades.
(2) Control the amount and consumption of protective slag:
The amount and consumption of protective slag directly affect its protective effect. During continuous casting, as the mold vibrates and the billet is pulled, the protective slag is continuously carried out of the mold, thus requiring continuous and batch-wise addition of new protective slag. Generally, the consumption of protective slag is 0.3-0.5 kg/t steel, but the actual consumption is affected by various factors such as casting speed and mold vibration parameters. To ensure effective coverage and good performance of the protective slag, the amount added needs to be precisely controlled according to the actual production situation, either through an automatic slag addition system or manual operation, to ensure that the thickness of the liquid slag layer remains stable within a suitable range. If the liquid slag layer is insufficient, a slag ring will form along the mold wall, blocking the downward channel of the meniscus, which may cause longitudinal cracks on the slab surface; conversely, if the liquid slag layer is too thick, its stability decreases, which can also cause defects such as longitudinal cracks on the slab surface.
Gas Protection Method
Principles and applications of argon protection
In the protection of the liquid surface in the crystallizer, gas protection is also a commonly used method. Among them, argon (Ar) is widely used in continuous casting production due to its inert properties. Argon does not react with active elements in molten steel (such as aluminum and titanium), and can form an inert gas protective layer on the surface of molten steel, effectively isolating it from air and preventing secondary oxidation and nitrogen accumulation in the molten steel.
Argon gas is commonly used to fill a sealed chamber between the tundish and the crystallizer, ensuring that the molten steel in the flow and on the crystallizer surface are within an argon protective atmosphere. A good protective effect is achieved by introducing argon gas into the flexible, sealed chamber between the tundish and the crystallizer, ensuring that the free oxygen content in the protective atmosphere is less than 1%. For some steel grades with less stringent nitrogen content requirements, nitrogen can also be used as a protective gas; however, its impact on the nitrogen content of the molten steel must be carefully controlled. In practice, precise control of the argon gas flow rate and pressure is necessary to ensure the formation of a stable and uniform gas curtain on the molten steel surface. If the argon gas flow rate is too low, an effective protective gas curtain cannot be formed, and air may still intrude into the molten steel; conversely, if the argon gas flow rate is too high, it may cause the molten steel surface to churn, increasing the risk of contact between the molten steel and air, and also resulting in energy waste.
Synergy between gas protection and other protection methods
Gas protection methods are typically used in conjunction with other mold surface protection methods to enhance the protective effect. When used in conjunction with protective slag, the argon gas curtain formed on the molten steel surface further prevents air from contacting the protective slag layer, reducing slag oxidation, thereby extending the service life of the protective slag and improving its protective performance. During continuous casting, when molten steel enters the mold from the tundish through the submerged entry nozzle, argon gas is also introduced at the joint between the submerged entry nozzle and the top nozzle to form an argon seal, preventing air from being drawn in through the joint and contacting the molten steel, thus preventing secondary oxidation. This multi-faceted gas protection, along with other protective methods such as protective slag, complements each other to provide more comprehensive and reliable protection for the molten steel within the mold.
