The application process of tundish coating agents directly impacts their metallurgical performance. Appropriate process parameters maximize the coating’s thermal insulation, oxidation protection, and inclusion adsorption properties. Improper operation can lead to performance failure and even deteriorate steel quality.
The following detailed analysis of their impact on metallurgical performance, focusing on addition method, dosage, particle size control, and process management, combined with literature data:
1. Influence of addition method on metallurgical effect
1. Timing of addition and distribution uniformity
If the covering agent is not added promptly (for example, delayed addition after pouring begins), the molten steel will be exposed to air and undergo secondary oxidation, leading to an increase in the total oxygen (T.O) content in the steel. One experiment showed that the T.O content of molten steel that was not covered promptly after pouring was 15% to 20% higher than that of molten steel that was covered promptly.
Uneven distribution during addition can expose areas of the molten steel, leading to localized oxygen and nitrogen enrichment. This can also result in poor spreading of the coating agent, uneven insulation thickness, and temperature drops as high as 5-10°C.
For example, manual spreading can easily cause accumulation, while mechanically applied coatings can form a continuous protective layer on the molten steel surface, reducing the secondary oxidation rate by 30%.
2. Layered adding process of double-layer covering agent
The double-layer coating (basic slag bottom layer + insulating slag top layer) requires a strict layering order: the bottom layer of alkaline slag is added first to quickly form a molten layer to absorb inclusions, while the top layer of insulating slag (such as carbonized rice hulls) is added later to enhance thermal insulation.
Reversing this order can make the bottom layer of insulating slag difficult to melt, reducing its inclusion absorption capacity by 40%. One steel plant demonstrated that correctly layering the slag can reduce heat loss in molten steel by 60% and increase inclusion removal by 25%.
2. Effect of addition amount on metallurgical effect
1. Relationship between thermal insulation effect and addition amount
If the amount of covering agent added is insufficient, the insulation layer becomes insufficiently thick, accelerating the temperature drop of the molten steel. Experimental results show that increasing the amount added from 0.5kg/t to 1.0kg/t reduces the temperature drop of the molten steel in the tundish from 15°C/h to 8°C/h. However, excessive addition (e.g., exceeding 1.5kg/t) results in an excessively thick molten layer, hindering the buoyancy of inclusions and increasing costs.
2. Critical addition amount of adsorption capacity
To effectively adsorb inclusions, the coating agent must form a molten layer of sufficient thickness (typically 5-10 mm). If the addition level is insufficient, the molten layer becomes thin, and the Al₂O₃ adsorption capacity is less than 20%. At an appropriate addition level (e.g., 1.0-1.2 kg/t), the adsorption capacity can reach 30-40%. For example, in Anshan Iron and Steel’s pipeline steel production, when the addition level is controlled at 1.14 kg/t, the Al₂O₃ content in the coating agent increases by 52% during the casting process, demonstrating a significant adsorption effect.
3. Influence of particle size and physical form on metallurgical effects
1. Effect of particle size distribution on melting rate
a) Coarse particle size (e.g., >1mm) slows the melting rate of the coating agent, making it difficult to quickly form a liquid slag layer and increasing the risk of secondary oxidation. Fine particle size (e.g., <0.1mm) increases dust generation and creates a thick sintered layer that is prone to crusting.
b) Optimizing the particle size (e.g., 0.2-0.8mm) can align the melting rate with the steel pouring rhythm. One experiment showed that coating agents in this particle size range shortened melting time by 20% compared to coarse particles and reduced crusting to below 5%.
2. Differences between hollow particles and solid particles
Hollow particle covering agents (such as spray-granulated products) have a low bulk density (0.78g/cm³) and offer 15% to 20% higher thermal insulation than solid particles. They also spread well, quickly covering the surface of molten steel. In one industrial trial, hollow particle covering agents reduced the temperature drop of molten steel by 4°C while increasing inclusion adsorption by 10%.
4. Detail control during use
1. Replenishment frequency and timing
During continuous pouring, covering agents must be added regularly to maintain the thickness of the slag layer. Failure to add covering agents promptly can lead to localized exposure. Data from one steel plant shows that adding covering agents every 30 minutes can keep the increase in nitrogen content in the molten steel within 5×10⁻⁶, while an interval exceeding 60 minutes can increase the nitrogen content by as much as 10×10⁻⁶.
2. Avoid mixing and contamination with other slag systems
Mixing ladle slag or drainage sand into the covering agent can disrupt its compositional stability. For example, SiO₂ from the drainage sand can enter the covering agent, reducing its basicity and resulting in a 25% drop in Al₂O₃ adsorption capacity. One steel plant has managed to control the amount of slag (less than 5kg/furnace) to keep the covering agent’s basicity fluctuation within ±0.2, ensuring stable adsorption.
3. Dynamic control of temperature and slag layer structure
The coating must form a three-layer structure: powder layer, sintered layer, and molten layer. The powder layer provides insulation, the sintered layer provides support, and the molten layer absorbs impurities. If the molten steel temperature is too high, the molten layer will be too thick, reducing insulation; if the temperature is too low, the molten layer will be too thin, resulting in insufficient adsorption capacity. By controlling the molten steel temperature in the tundish within a certain range, the three-layer structure can be optimized.
The application process of the tundish covering agent directly affects the metallurgical effects such as heat preservation, oxidation prevention and inclusion adsorption by affecting its spreadability, melting rate, slag layer structure and composition stability. Key process parameters include:
Adding method: evenly distribute the material and add in layers (double-layer covering agent) to reduce secondary oxidation;
Addition amount: 1.0~1.2kg/t is appropriate to balance the needs of heat preservation and adsorption;
Particle size: 0.2~0.8mm or hollow particles can optimize melting and heat preservation performance;
Process management: regular replenishment, avoidance of contamination, and control of molten steel temperature to maintain a stable three-layer structure.
