Refractory Materials for Tundish Linings and Precautions for Preheating

Refractory Materials for Tundishes

Tundish refractory linings comprise various components, including the vessel body, lid, upper and lower nozzles, stopper rods, slag-retaining weirs and dams, flow stabilizers, and impact pads. Refractory materials for tundishes are categorized into lining materials (working layer, permanent layer, and thermal insulation layer) and materials for continuous casting components and precast shapes. Due to the varying locations and operational requirements within the tundish, the physicochemical properties and performance characteristics of the refractory materials differ accordingly. High-quality refractory materials are essential for ensuring the service life of continuous casting tundishes; we can provide a full range of tundish refractory materials tailored to your specific requirements. Zircon bricks or precast blocks are utilized in the area of ​​the tundish bottom subject to molten steel impingement.

Refractory Materials for Tundishes

The tundish lining primarily consists of a thermal insulation layer, a permanent layer, and a working lining. Clay bricks or insulation boards are used for the thermal insulation layer, while low-cement or ultra-low-cement castables (with 60%–80% alumina content) are typically used for the permanent layer. The working lining must meet the following criteria:

(1) Resistance to erosion by molten steel and slag, ensuring a long service life.

(2) Good thermal shock resistance, preventing cracking or spalling upon contact with molten steel.

(3) Low thermal conductivity and low thermal expansion coefficient, providing the lining with effective thermal insulation properties and good structural integrity.

(4) No contamination of the molten steel during the casting process.

(5) Ease of assembly and disassembly.

Construction Technology for Continuous Casting Tundish Refractories

With the evolution of refractory materials, the primary construction methods for tundish refractories are composite masonry (using refractory bricks) and monolithic casting (using bulk materials); the adage "30% material, 70% installation" underscores the critical importance of the construction process. The installation teams at Rongsheng Refractories employ various techniques to meet diverse customer requirements. Rongsheng Refractories recommends the monolithic casting process for continuous casting tundishes, as it enhances sealing performance and extends the vessel's service life. The lining thickness is typically 100–120 mm for small-to-medium tundishes and 150–200 mm for large tundishes.

Characteristics of using refractory slag-blocking walls in the working lining of steelmaking tundishes.

Refractory materials for tundish slag-blocking weirs: covering materials such as impact pads and slag-blocking weirs, their material composition, physicochemical properties, and potential drawbacks, as well as the impact of molten steel flow velocity on the tundish.

Theoretical residence time is calculated based on the molten steel flow rate and the volume of the tundish. However, the flow of molten steel within the tundish is non-uniform, with varying velocities across different zones. In particular, stagnant zones of inactive molten steel exist at the bottom of the tundish, making it difficult for inclusions to float to the surface. Consequently, the actual residence time of the molten steel is shorter than the theoretical calculation. To maximize the effective utilization of the tundish volume and promote the flotation of inclusions, measures such as installing slag-blocking walls and dams are implemented with the following objectives:

1. Eliminating dead zones at the bottom of the tundish.

2. Improving the flow trajectory of the molten steel to promote flow at the steel-slag interface, thereby shortening the distance inclusions must travel to float up and facilitating their absorption by the slag.

3. Confining the strong vortexes generated by the ladle stream's impact to a localized area, thereby preventing turbulent diffusion that could cause surface waves and the entrapment of slag into the molten steel.

What points should be considered during tundish preheating?

For the tundish preheating process, it is generally ideal to rapidly heat the vessel to 1000–1100°C within 1–2 hours prior to the start of casting. This approach saves energy and facilitates operations. However, due to factors such as production scheduling, preheating times are often excessively long, creating potential risks.

In slab continuous casting, the tundish is typically equipped with a monolithic alumina-carbon stopper rod or a monolithic alumina-carbon submerged entry nozzle (SEN). Excessive preheating times reduce the strength of these components. If the anti-oxidation coating on the product surface is defective, oxidation and structural loosening occur; this not only further reduces strength but also shortens the service life. Past experience indicates that for submerged nozzles—once the wall thickness has stabilized—extending service life depends critically on whether the surface undergoes oxidation.

The pattern of strength variation is as follows: product strength decreases as the preheating temperature rises, reaching a minimum in the 500–600°C range. As the temperature continues to rise, strength increases, but it begins to decline again upon reaching approximately 1300°C. Therefore, rapid preheating is required to reach the target temperature.

Regarding quartz-based submerged entry nozzles, prolonged exposure to high-temperature preheating should be avoided; they require either no preheating or only low-temperature preheating. Prolonged high-temperature exposure causes crystallization in quartz nozzles, reducing thermal stability and potentially leading to cracks on the inner wall. This can result in breakouts or nozzle rupture during casting (though this also depends on the inherent quality of the nozzle).

For billet continuous casting tundishes, insulating boards are commonly used as liners, and the tundish itself is often not preheated; some steel plants may apply only a low-intensity flame, serving merely to dry the vessel. Billet tundishes typically feature 3 to 6 microporous nozzles (with diameters of 3–6 mm). These nozzles require separate, thorough preheating; otherwise, they may explode during use. Due to the exceptionally poor thermal stability of zirconia-based products, inadequate preheating can lead to cracking.