Classification And Application Configuration Of Refractories For Tundishes

Tundish Structure

The tundish is a transitional device used to receive molten steel from the ladle and direct it into the mold during the continuous casting process. It not only serves as a storage and distribution unit for molten steel, but also acts as a refining vessel. The tundish stabilizes the flow of molten steel, reducing the scouring effect of the flow on the solidified shell in the mold. Additionally, it ensures a reasonable flow field and adequate residence time for molten steel inside the tundish. This promotes uniform temperature distribution, facilitates the separation and floating of non-metallic inclusions, and guarantees molten steel cleanliness. It also supports multi-heat continuous casting, making it a critical component for improving steel product quality and continuous casting efficiency. The structure is illustrated in Figure 1.

Main Functions of the Tundish

(1) Distribute molten steel

For multi-strand continuous casters, the tundish distributes molten steel to each individual mold.

(2) Stabilize the molten steel flow

It reduces the hydrostatic pressure of molten steel, maintains a stable liquid level inside the tundish, and ensures a steady flow of molten steel into the mold.

(3) Store molten steel

During ladle changes in multi-heat continuous casting operations, it allows uninterrupted casting at a constant withdrawal speed, thereby creating favorable conditions for multi-heat continuous casting.

(4) Purify molten steel

Over an extended casting period, the tundish maintains the temperature of the molten steel at a nearly constant level, promotes the further floating of inclusions in the molten steel, prevents contact between the molten steel and air, and avoids the absorption of oxygen and nitrogen.

Refractories for Tundishes

Functionally, tundish refractories can be classified into three categories:

Lining materials

These are primarily composed of the insulation layer, permanent layer, and working layer.

Flow-stabilizing components

These include slag weirs, slag baffles, impact plates, and other parts.

Flow-control components

This category includes stopper rods, slide gates, submerged entry nozzles (SENs), and metering nozzles.

For billet continuous casting, the core of high-efficiency continuous casting lies in high withdrawal speed. This requires maximizing the continuous operation rate and casting speed of the caster, while ensuring that molten steel quality requirements are met. The goal is to achieve a high strand outlet temperature and excellent billet quality. These technical indicators place two demands on tundish refractories: high performance and long service life.

Performance Requirements for Tundish Refractories

Excellent high-temperature resistance

The temperature of molten steel in the tundish is usually above 1500°C and can even reach 1600°C. Therefore, tundish refractories should have excellent high-temperature resistance to ensure stability and service life under such conditions.

Superior corrosion resistance

Molten steel contains a significant amount of impurities, such as iron oxide and calcium oxide, which can severely corrode refractories. Therefore, tundish refractories should possess superior corrosion resistance to guarantee their service life.

Good thermal shock resistance

During the continuous casting process, the temperature of molten steel in the tundish fluctuates significantly, which can cause refractories to crack or even rupture. Therefore, tundish refractories should have good thermal shock resistance to maintain stability under temperature fluctuations.

Favorable air permeability

Tundish refractories should have favorable air permeability to ensure the smooth discharge of molten steel and the escape of gases, thereby maintaining the stability of the continuous casting process.

Easy construction and maintenance

Tundish refractories should be easy to construct and maintain, reducing production costs and improving production efficiency.

Composition of Tundish Lining Refractories

The composition of tundish lining refractories generally includes the following components:

(1)Insulation layer (10–30 mm)

This layer is in direct contact with the steel shell of the tundish and is usually made of asbestos boards, insulating bricks, or lightweight castables. Aluminosilicate fiber boards offer optimal performance, featuring low thermal conductivity and easy installation.

(2)Permanent layer (100–200 mm)

Adjacent to the insulation layer, this layer is typically made of fireclay bricks. Monolithic permanent linings are the most widely used type, with castables usually being high-alumina or mullite self-flow castables.

(3)Working layer (20–50 mm)

As the key component in direct contact with molten steel, this layer is commonly constructed with insulating boards or coatings. Insulating boards are usually made of silica-based, magnesia-based, or forsterite-based materials, while coatings are formulated with magnesia-based, magnesia-chrome-based, or magnesia-calcia-based materials. The coatings can be applied either by mechanical spraying or manual troweling.

(4)Nozzle bricks

Embedded in the bottom of the tundish, these bricks serve as fixtures for installing tundish nozzles and are typically made of high-alumina materials.

(5)Tundish bottom

The material of the tundish bottom is essentially the same as that of the working layer. The working layer of the tundish bottom, especially in the area subjected to molten steel impact, is prone to damage and thus requires excellent corrosion and wear resistance. It is typically constructed with high-alumina bricks, dense high-alumina bricks, alumina-chrome bricks, or specially manufactured large-sized high-strength tar-bonded magnesia bricks. Zircon bricks or precast blocks are used to reinforce the impact zone.

(6)Tundish cover

Covering the tundish, it functions to preserve heat and prevent molten steel splashing. The cover is usually made of fireclay-based or high-alumina-based castables.

(7)Slag dams (weirs)

Installed inside the tundish, these dams can be either single-wall or double-wall structures. They are usually made of high-alumina bricks or precast blocks, and their primary purpose is to block slag. To further improve the cleanliness of molten steel, molten steel filters can also be installed on the slag dams.

Commonly Used Refractories for Tundishes

Magnesia-chrome bricks

Magnesia-chrome bricks are refractories with excellent corrosion resistance, making them suitable for tundishes handling molten steel at relatively high temperatures. However, they exhibit poor thermal shock resistance, which makes them prone to cracking, and they also come with a high price tag.

High-alumina bricks

A common type of refractory, high-alumina bricks feature good high-temperature resistance and thermal shock resistance. However, their corrosion resistance is relatively weak, limiting their application to tundishes handling lower-temperature molten steel.

Zircon bricks

Zircon bricks offer outstanding corrosion resistance and high-temperature resistance, making them ideal for tundishes processing high-temperature molten steel. However, they are expensive and pose greater construction challenges.

Silicon carbide bricks

Silicon carbide bricks boast excellent corrosion resistance and high-temperature resistance, making them suitable for tundishes with high-temperature molten steel. However, their thermal shock resistance is poor, making them susceptible to cracking.