In the continuous casting process, the mold powder’s “three-layer structure” (raw slag layer, sintered slag layer, and liquid slag layer) is crucial for its full functionality. Through this three-layer structure, mold powder must fulfill five key functions: air insulation, preventing secondary oxidation of the molten steel, lubricating the ingot, absorbing non-metallic inclusions, and evenly transferring heat to the mold. The key to achieving a stable “three-layer structure” lies in precisely controlling the melting rate of the mold powder, ensuring that the slag added to the molten steel surface does not melt completely all at once, but rather gradually melts. This article will comprehensively analyze the technical path to achieving a three-layer structure of mold powder, focusing on key methods for regulating the melting rate and key parameter control points.
1.Carbon Particles: The Core Regulator for Controlling the Melting Rate
Controlling the melting rate of mold powder primarily relies on the addition of carbon particles to the slag as a melting rate regulator. As a high-temperature resistant material, carbon’s unique mechanism of action provides the foundation for the formation of the “three-layer structure”: Ultra-fine carbon powder adsorbs onto the surface of mold slag particles, separating slag particles that might otherwise contact and fuse. This directly inhibits slag agglomeration and melting, effectively slowing the melting rate and creating conditions for layered melting.
The effectiveness of carbon particles in controlling the melting rate depends entirely on their type and amount. Properly combining the two is key to avoiding excessively fast or slow melting rates and ensuring a stable three-layer structure.
(I) Carbon Particle Type Selection: Adapting to Melting Rate Requirements in Different Temperature Zones
Currently, the most commonly used carbon blending materials in industry are graphite and carbon black. Due to differences in structure, particle size, and oxidation properties, these two materials exhibit distinct melting rate control capabilities in different temperature zones. Their selection requires tailoring to the temperature distribution characteristics of the mold slag layer.
Graphite: An Efficient Rate-Controlling Material for High-Temperature Zones
Graphite has a granular structure and large particle size, typically ranging from 60-80μm. Its notable characteristics are its relatively weak ability to separate and inhibit slag fusion, but its high oxidation starting temperature (approximately 560°C) and slow oxidation rate. This characteristic enhances its ability to control the melting rate in the high-temperature region of the protective slag layer (the higher-temperature region near the molten steel). Even at high temperatures, graphite oxidizes slowly and maintains its separating function, preventing rapid melting of the slag in the high-temperature region and ensuring the formation of a gradient between the sintering layer and the liquid slag layer.
Carbon Black: A Precise Rate Control Material for Low-Temperature Regions
Carbon black has an amorphous structure and extremely fine particles, measuring only 0.06-0.10μm, much smaller than graphite. Its advantages lie in its strong separation and retardation of slag particles, and its rapid onset of action at low temperatures. However, its disadvantages are also significant: its low oxidation starting temperature (approximately 500°C) and rapid oxidation rate make it easily consumed in the high-temperature region. Even increasing its addition level has only a limited effect on improving the melting rate in this region. Therefore, carbon black is more suitable for the low-temperature region of the mold slag layer (the cooler area near the liquid surface), preventing the slag from melting prematurely at low temperatures and allowing time for the transition from the raw slag layer to the sintered layer.
(II) Controlling the Quantity of Carbon Particles: Determining the Thickness Balance of the Three-Layer Structure
The amount of carbon particles added directly affects the thickness ratio between the sintered layer and the liquid slag layer and must be strictly controlled within a reasonable range. In industrial production, the amount of carbon powder added is typically 4%-7% of the total mold slag mass. This range, proven in extensive practice, is the “golden range” for achieving a perfect thickness between the sintered and liquid slag layers. Deviating from this range directly undermines the stability of the three-layer structure.
Insufficient Carbon Powder: Excessively Fast Melting Rate, Excessively Thick Liquid Slag Layer
When the carbon powder addition is less than 4%, the carbon particles will burn out prematurely before the slag layer temperature reaches the temperature at which the mold slag begins to sinter, due to their oxidation rate exceeding their generation rate. Without the separating effect of the carbon particles, the slag will rapidly agglomerate and melt, resulting in an overly developed sintered layer (abnormally increased thickness) and a significantly thicker liquid slag layer. In this case, the original slag layer is rapidly consumed, and the three-layer structure gradually degenerates into a two-layer structure of “sintered layer + liquid slag layer,” significantly reducing the mold slag’s thermal insulation and lubrication capabilities.
Excessive Carbon Powder: Slow Melting Rate, Thin Sintered Layer
If the carbon powder addition exceeds 7%, the opposite problem occurs: even after the mold slag has completely melted to form a liquid slag layer, some unoxidized carbon particles may remain. Excessive carbon particles continuously hinder the slag sintering process, causing the sintered layer to shrink (thickness drops to less than 5mm or even disappear completely). Direct contact between the original slag layer and the liquid slag layer disrupts the three-layer structure. The residual carbon particles reduce the fluidity of the liquid slag layer, preventing it from evenly covering the molten steel surface, which can easily lead to surface defects in the ingot.
Appropriate Carbon Powder Content: Stable Three-Layer Structure and Efficient Functionality
When the carbon powder addition level is controlled within the optimal range of 4%-7%, a balanced state is achieved in the sintered layer, with some carbon particles burned out and some remaining. The burned-out areas provide conditions for slag sintering, forming a stable sintered layer. The remaining carbon particles continue to hinder the rapid melting of the unsintered slag, ensuring a stable thickness in the original slag layer. Ultimately, a three-layer structure with optimal thicknesses is formed: the original slag layer (15-20mm), the sintered layer (10-15mm), and the liquid slag layer (10-16mm), enabling the five key functions of the mold slag to be fully utilized.
2. Mold Flux Addition Amount and Timing: Maintaining the Dynamic Stability of the Three-Layer Structure
In addition to carbon particle control, properly controlling the amount and timing of mold slag addition, depending on the different stages of the continuous casting process, is another key to maintaining the long-term stability of the “three-layer structure.” During the continuous casting process, the slag layer continuously decreases due to factors such as lubrication depletion and inclusion absorption. If the addition amount and timing are inappropriate, the already formed three-layer structure will gradually become unbalanced. Therefore, a precise addition strategy must be formulated in stages.
(I) Phase I: Initial Layer Formation: High Addition Lays the Structural Foundation
During the initial stage of continuous casting (usually from the start of continuous casting to stable casting), the amount of mold slag added should be appropriately increased to quickly form a three-layer structure of the required thickness on the molten steel surface. According to relevant patent literature and industrial practice data, the amount of mold slag added during this stage should be controlled at “0.5-0.6 kg per ton of molten steel.” This addition ensures: the raw slag layer quickly accumulates to 15-20mm, providing raw material for insulation and subsequent sintering; the sintered layer, controlled by carbon particles, forms a stable thickness of 10-15mm, acting as a “transition buffer”; and the liquid slag layer reaches a thickness of 10-16mm, quickly covering the molten steel surface, isolating it from air and beginning to lubricate the ingot.
(II) Second Stage: Stable Operation, Low Addition to Maintain Structural Balance
When continuous casting enters the stable operation stage, the rate of mold slag consumption stabilizes. At this point, the addition rate should be reduced to prevent excessive increase in slag layer thickness, which could lead to structural imbalance. The addition rate during this stage should be adjusted to 0.3-0.4 kg per ton of molten steel. This dosage just replenishes the slag layer while preventing excessive accumulation of the raw slag layer or excessive melting of the liquid slag layer. It’s worth noting that during stable operation, the “three-layer structure” will experience slight adjustments. In some cases, the thickness of the sintered layer will drop below 5mm, forming a two-layer structure consisting of a “sintered layer + liquid slag layer.” In areas with extremely low slag consumption, only the liquid slag layer may remain. This dynamic adjustment is normal; as long as the liquid slag layer continues to function, continuous casting stability is ensured.
3. Summary: Multi-dimensional Collaborative Control is the Core Guarantee of the Three-Layer Structure
Achieving the “three-layer structure” of mold powder relies on a multi-dimensional synergy: controlling the melting rate through the type and quantity of carbon particles, and matching the addition amount and timing to the process stages. This ensures that the slag powder melting process is precisely aligned with continuous casting requirements. The selection and addition amount of carbon particles are the “core variables” that determine the initial formation of the three-layer structure, while the addition amount and timing of mold powder are the “regulating variables” that determine the long-term stability of the three-layer structure. Only by effectively combining these two factors can the mold powder fully utilize its insulation, lubrication, and temperature control functions, ensuring efficient and high-quality continuous casting. In actual applications, dynamic adjustments are also required based on factors such as the characteristics of steel grades (such as the difference in melting points between low-carbon steel and high-carbon steel), the type of crystallizer (such as curved crystallizer and straight-arc crystallizer), etc. By continuously optimizing parameters, precise control of the three-layer structure can be achieved, ultimately improving the quality of the casting and reducing production costs.
