In the continuous casting process, the copper mould tube is a critical piece of equipment for solidifying and shaping molten steel. Mold level protection is crucial for ensuring a smooth continuous casting process and improving the quality of the ingot. Good mold level protection effectively isolates the air, prevents secondary oxidation of the molten steel, reduces the formation of inclusions, and stabilizes the solidification process, profoundly impacting the internal and surface quality of the ingot. The following details common methods for mold level protection.
Application of Mold Powder
Mechanism of Mold Powder
Mold powder plays a central role in protecting the mold liquid level. Once added to the molten steel surface in the mold, it undergoes a complex series of physical and chemical changes. First, the mold flux near the molten steel surface rapidly absorbs heat from the high-temperature molten steel, forming a slag layer. Above this layer is a sintered layer (also known as a transition layer), and on top is a loose powdery layer, forming a typical three-layer structure.
(1) Isolating from Air and Preventing Secondary Oxidation: The slag layer formed after the mold flux melts evenly covers the surface of the molten steel, acting like a protective “suit” for the molten steel. This effectively isolates the molten steel from the air, preventing oxygen, nitrogen, and other gases in the air from coming into contact with the molten steel, thereby preventing secondary oxidation. This function is particularly critical for steel grades containing high levels of easily oxidizable elements, such as aluminum-killed steel and stainless steel. If secondary oxidation occurs in molten steel, it can lead to the formation of numerous oxide inclusions, such as aluminum oxide (Al2O3). These inclusions can severely impact the steel’s performance, reducing its strength, toughness, and fatigue life. They can also cause surface defects, lowering product yields.
(2) Thermal Insulation: The three-layer structure of mold slag reduces radiative heat loss from the molten steel and reduces superheat. During mold slag operation, a thick layer of powdered slag, known as “black slag operation,” is typically required above the liquid slag layer. This layer further enhances the thermal insulation effect, raising the mold meniscus temperature and reducing the formation or excessive growth of slag rings. Especially when casting high-carbon steel, good thermal insulation properties help improve lubrication conditions for the ingot and prevent surface defects such as cracks. The thermal insulation properties of the mold 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 mold slag.
(3) Absorbing Inclusions in Molten Steel: To prevent floating inclusions from the molten steel from being drawn into the solidifying shell and causing surface or subsurface defects in the ingot, the molten slag layer formed by the mold slag must be capable of absorbing and assimilating floating non-metallic inclusions. When inclusions float to the slag layer, the mold slag captures and dissolves them, thereby purifying the molten steel and improving the purity of the ingot. For example, in the production of high-quality steel for automotive panels, the mold slag’s effective absorption of inclusions can significantly reduce surface defects and improve the surface quality of the steel.
(4) Lubricating and Improving Heat Transfer through the Slag Film: The slag film formed between the mold wall and the solidifying shell acts as a lubricant, reducing drawing resistance and preventing adhesion between the shell and the mold wall. Furthermore, a suitable slag film thickness and uniformity can improve heat transfer uniformity within the mold. The slag film reduces the heat transfer rate in the upper portion and increases it in the lower portion, resulting in a more balanced temperature distribution during the solidification process. This promotes uniform solidification structure and improves ingot quality. Generally, the thickness of the slag film ranges from 0.1 to 1.5 mm. To ensure optimal slag film performance, the mold slag must be melted at an appropriate rate, maintaining a 6-15 mm thick layer of liquid slag above the molten steel surface in the mold.
Selection and Maintenance of Mold Slag
(1) Selecting the appropriate mold slag based on the steel grade: Different steel grades require different mold slag properties. For low-carbon steel, the mold slag must have low basicity and good lubricity to ensure a high surface quality. For high-carbon steel, the mold slag must have high thermal insulation and suitable melting characteristics. When producing special steel grades, such as those containing microalloying elements like titanium and niobium, the mold slag must also possess appropriate chemical stability to avoid adverse reactions with these alloying elements. Therefore, steel companies need to select specialized mold slag based on the characteristics of the steel grades they produce to meet the continuous casting process requirements of each steel grade.
(2)Control the amount and consumption of mold slag: The amount and consumption of mold slag directly impacts its protective effect. During the continuous casting process, mold slag is continuously removed from the mold as the mold vibrates and casting progresses, necessitating continuous, batch-by-batch addition of new mold slag. Generally, mold slag consumption is 0.3-0.5 kg/t of steel, but actual consumption is affected by various factors, such as casting speed and mold vibration parameters. To ensure effective mold slag coverage and optimal performance, the amount of mold slag added must be precisely controlled based on actual production conditions, either through an automatic slag feeding system or manual operation, ensuring that the thickness of the liquid slag layer remains within an appropriate range. If the slag layer is insufficiently thick, a slag ring will form along the mold wall, blocking the downward path of the meniscus and potentially causing longitudinal cracks on the slab surface. Excessively thick slag reduces its stability, similarly causing defects such as longitudinal cracks on the slab surface.
Gas Shielding Methods
Principles and Applications of Argon Shielding
Gas shielding methods are a common method for protecting the mold surface. Argon (Ar) is widely used in continuous casting due to its inert properties. Argon does not react with reactive elements in molten steel (such as aluminum and titanium), forming a protective layer of inert gas on the molten steel surface, effectively isolating it from air and preventing secondary oxidation and nitrogen enrichment.
A sealed chamber between the tundish and the mold is often filled with argon, creating an argon protective atmosphere around the molten steel surface. Argon is introduced into the flexible sealed chamber between the tundish and the mold, ensuring that the free oxygen content in the protective atmosphere is less than 1% to achieve effective shielding. Nitrogen can also be used as a shielding gas for some steel grades with less stringent nitrogen requirements, but its impact on the molten steel’s nitrogen content must be carefully controlled. In practice, precise control of the argon flow rate and pressure is required to ensure a stable and uniform gas curtain on the molten steel surface. If the argon flow rate is too low, an effective protective gas curtain cannot be formed, allowing air to intrude into the molten steel. Excessive argon flow can cause the molten steel surface to churn, increasing the risk of air contact and wasting energy.
Synergy of Gas Shielding with Other Shielding Methods
Gas shielding is often used in conjunction with other mold surface protection methods to enhance the protective effect. When combined with mold slag, the argon gas curtain formed on the molten steel surface further prevents air from contacting the mold slag layer, reducing oxidation and thus extending the slag’s service life and improving its protective properties. During the continuous casting process, as molten steel enters the mold from the tundish through the submerged nozzle, argon is also introduced into the joint between the submerged nozzle and the upper nozzle, forming an argon seal to prevent air from being drawn through the joint and contacting the molten steel, leading to secondary oxidation. This multi-faceted gas shielding complements other shielding methods, such as mold slag, to provide more comprehensive and reliable protection for the molten steel in the mold.
