Common Problems of Steelmaking Tundish Refractory Material Configuration and Lining

To control the molten steel temperature precisely, adjust trace alloy elements accurately, and perform calcium treatment to improve inclusions in the tundish, it’s essential to recognize that the tundish has distinct requirements compared to the ladle due to differences in capacity and molten steel temperature. Consequently, the refractory materials used in the tundish are specialized. Regardless of the tundish type, these refractory materials must meet the following requirements:

1. The lining material must resist corrosion from molten steel and slag, ensuring a long service life.

2. The material should exhibit strong thermal shock resistance, preventing cracking upon contact with molten steel.

3. Low thermal conductivity and minimal thermal expansion are essential to provide effective heat insulation and maintain the integrity of the tundish lining.

4. The lining material should minimize contamination to the molten steel during pouring, preserving steel quality.

5. The design and structure of the lining material should allow for easy construction and disassembly.

Intermediate tundish types include:

1. High-Temperature Tundish  

Designed for specific metallurgical processes, this tundish uses magnesia bricks for its lining and is preheated to around 1500°C.

2. Hot Tundish  

Commonly used in steel plants, it features a lining of fired bricks, unfired bricks, or castables, preheated to 800–1100°C before pouring.

3. Cold Tundish  

Uses insulation boards as lining and does not require preheating before pouring.

The optimal tundish type and lining material depend on factors such as the steel grade, smelting method, preheating conditions, and the steel plant's capabilities.

The tundish lining comprises several key layers and components:

1. Insulation Layer (10–30 mm):  

Located next to the tundish steel shell, this layer is typically made from asbestos board, insulating brick, or lightweight castable.

2. Permanent Layer (100–200 mm):  

Positioned against the insulation layer, this layer is usually constructed from clay bricks.

3. Working Layer (20–50 mm):  

This layer, which contacts the molten steel, is made from high-alumina bricks, alkaline bricks (e.g., magnesia bricks), siliceous or magnesia insulation panels, forsterite insulation panels, and coatings like magnesia or magnesia-chrome. Castables are also commonly used.

4. Block:  

Inlaid at the tundish bottom to house the nozzle, typically made of high-aluminum material.

5. Bottom Layer:  

The material is generally similar to that of the working layer.

6. Cover:  

The tundish cover retains heat and prevents steel splashing. It is typically made from clay bricks or castables.

7. Slag Retaining Wall:  

Built inside the tundish to block slag and enhance molten steel cleanliness. High-alumina materials are used, with options for brick construction or prefabricated blocks. Some include a molten steel filter to further reduce inclusions.

Challenges with tundish refractory linings can arise from material quality or on-site construction, necessitating careful observation and analysis for resolution.

Dry Material Lacks Strength  

For effective use, the dry material of the tundish working layer must attain sufficient low-temperature strength, achieved primarily through vibration and baking. However, after vibration and demolding, some areas may still lack strength, risking collapse and affecting continuous casting. Observations have identified the main reasons for this issue:

1. Baking Issues:  

The tundish baking devices in steel plants, typically gas-based, can accumulate tar in the pipelines or suffer burner damage over time, leading to uneven baking and weak spots.

2. Damp Material:  

Dry material consists of about 70% particles and 30% fine powder. This fine powder, containing magnesia and binders, is highly absorbent. When it absorbs moisture, the magnesia and binder can degrade, preventing effective bonding during baking.

3. Uneven Binder Mixing:  

During production, particles, fine powders, and binders must be thoroughly mixed. Human error or equipment issues may lead to uneven mixing, leaving some material without sufficient binder, which reduces strength after baking.

Solution:  

To address these issues, regularly clean the gas pipelines to remove tar and dust, replace any damaged burners, and ensure that dry materials are properly dried and thoroughly mixed.

Turbulator Floating Issue  

Turbulators are essential in continuous casting, as they stabilize steel flow and protect the impact area. Made of magnesia-based refractory material, they are part of the slag-retaining wall system. During multi-furnace continuous casting, turbulators may occasionally float on the molten steel surface, which disrupts steel flow stabilization, impacting molten steel quality and production safety. This floating occurs because the density of the refractory material is lower than that of molten steel, causing it to rise when immersed.

Floating can manifest in two ways:  

1. Frame Floating:  

The turbulator's magnesia material expands in high-temperature molten steel, leading to potential cracking. The connection between the turbulator frame and impact plate is a weak point, making it susceptible to separation.

2. Overall Floating:  

This occurs if the bottom of the turbulator is not properly aligned with the working layer. Molten steel may seep into gaps between the turbulator and the working layer, causing the entire turbulator to float.

Solution:  

To prevent floating:

1. Adjust the turbulator formula to control high-temperature expansion.

2. Apply a layer of refractory powder on the working layer surface during installation to ensure there are no gaps between the turbulator and the working layer.

Nozzle Cracking and Steel Seepage  

During pouring, tundish nozzles with zirconium cores are prone to cracking, leading to steel seepage. This often results in production interruptions or unexpected shutdowns in continuous casting. The main cause of cracking is the zirconium core's limited thermal shock resistance.

Solution:  

1. Avoid excessively high bulk density in the zirconium core, as higher density reduces thermal shock resistance.

2. Increase the nozzle body thickness to further prevent steel seepage.

Ladle Casing Fracture  

The ladle casing, situated between the ladle nozzle and the tundish, plays a crucial role in preventing molten steel from splashing and oxidizing as it flows into the tundish. However, casing fractures are a common issue. This can result from two main factors: (1) poor thermal shock resistance of the casing, causing cracks; and (2) external force applied to the casing when it adheres tightly to the ladle nozzle, resulting in breakage.

Solutions:  

1. Use materials with a low thermal expansion coefficient and high elasticity to improve thermal shock resistance.

2. When separation between the casing and nozzle is difficult, apply force to the upper part of the casing rather than the lower part to minimize risk of fracture.

Other Common Issues  

Additional challenges identified through industry feedback include dry material collapse, working lining cracks, nozzle damage, slag wall peeling, and steel infiltration. Each of these requires attention to maintain efficiency and safety in steelmaking operations.