Common problems and solutions for refractory materials used in tundishes

This article introduces some common problems encountered in the use of refractory materials for continuous casting tundishes, analyzes the causes of these problems, and proposes solutions and methods to ensure stable continuous casting production.

The tundish is a crucial vessel in the continuous casting process for steelmaking, receiving high-temperature molten steel. Molten steel is poured from the ladle into the tundish and then enters the crystallizer to form steel billets, which is essential for ensuring clean steelmaking and continuous casting across multiple heats. Various refractory materials are required in the production and use of tundishes, including dry refractory materials, nozzles, slag retaining walls, and the three main components of continuous casting. Problems such as lack of strength in the working lining, nozzle cracking and steel leakage, and turbulence in the flow apparatus are common in tundish refractory materials during use, seriously affecting stable continuous casting production. Therefore, effectively addressing these problems is crucial.

1. Configuration of refractory materials for tundishes

The tundish refractory materials include the permanent layer castable, the working layer dry refractory, the nozzles and bedding bricks, the slag retaining wall system, and the three main components of continuous casting.

1.1 Permanent Layer

The permanent layer lies between the working layer and the tundish shell, not in direct contact with molten steel or slag. Its main function is insulation and protection of the cladding shell. It is generally cast with alumina-magnesia castable.

1.2 Working Layer

The working layer is in direct contact with molten steel and slag, and must withstand the scouring of molten steel and the erosion and penetration of slag. The working layer mostly uses magnesia dry refractory, which has good slag resistance, is easy to construct, easy to disassemble, and also helps to purify the molten steel.

1.3 Nozzles and Bedding Bricks

The tundish nozzle is the channel through which molten steel enters the crystallizer from the tundish, and must withstand the scouring of molten steel and rapid temperature changes. The bedding bricks protect and fix the nozzle, and do not come into direct contact with the molten steel. The tundish nozzle is mostly composed of a zirconium core and a high-alumina billet, while the seat bricks are generally made of high-alumina material.

1.4 Slag Retaining Wall Series

The refractory materials for the slag retaining wall series mainly include slag retaining walls, side plates, back plates, and turbulence generators, which must withstand the scouring of molten steel and the erosion and penetration of steel slag. They are generally made of magnesia materials.

1.5 Three Major Components of Continuous Casting

The three major components of continuous casting include the ladle sleeve, stopper rod, and submerged entry nozzle, which must withstand the scouring of molten steel and the erosion and penetration of steel slag. The stopper rod is used to control the flow rate of molten steel during casting, while the ladle sleeve and submerged entry nozzle are used to protect the casting, isolate air, and prevent secondary oxidation of the molten steel. The three major components of continuous casting are mostly made of alumina-carbon materials, sometimes with the addition of some zirconium oxide.

2. Common problems and solutions of refractory materials in tundishes

2.1 Dry Material Lacks Strength

Before the tundish is put into use, the dry material in the working layer needs to have a certain low-temperature strength, which is mainly obtained through vibration and baking. After the dry material is demolded through vibration and baking, local areas often show no strength or low strength, which can easily cause the ladle to collapse and affect the safe production of continuous casting. Long-term observation and analysis have revealed the following main reasons for the lack of strength or low strength in dry-mixed tundishes:

1) Baking problems: The tundish baking equipment used in steel plants is a gas-fired baker. Long-term use can cause a large accumulation of tar in the pipes or damage to the burners, resulting in poor local baking and thus lack of strength or low strength.

2) Moisture absorption of dry-mixed tundishes: Dry-mixed tundishes consist of 70% granules and 30% fine powder. The fine powder includes magnesia and binder. Due to its high specific surface area, the fine powder easily absorbs moisture. The magnesia and binder hydrate and deteriorate after absorbing moisture, causing the binder to fail and preventing the formation of bonding strength during baking.

3) Uneven mixing of binder: During the production of dry-mixed tundishes, various granules, fine powder, and binders need to be mixed evenly. Sometimes, human factors or the mixing equipment can cause uneven mixing, resulting in some dry-mixed tundishes lacking binder or containing very little binder, leading to localized areas of lack of strength after baking and demolding.

Solutions: First, ensure the baking effect of the refractory burner by regularly purging the gas pipeline to remove tar and dust, and replace damaged burners promptly. Second, ensure the dry materials are dry and the mixing is uniform.

2.2 Turbulent Flow Floating

The turbulent flow refractory mainly plays a role in stabilizing the steel flow and protecting the impact zone during continuous casting. It belongs to the slag retaining wall series of refractory materials and is generally made of magnesia. During multi-furnace continuous casting, the turbulent flow refractory sometimes floats on the surface of the molten steel, failing to stabilize the steel flow and protect the impact zone, which is detrimental to the quality of the molten steel and safe production. The turbulent flow refractory floats because the density of the refractory material is less than that of the molten steel; when immersed in molten steel, it will float. Turbulent flow refractory floating includes two situations: the floating of the turbulent flow rim and the floating of the entire refractory. The floating of the turbulence generator frame is due to the excessive expansion coefficient of the magnesium material under high-temperature molten steel immersion, which easily causes cracking. The contact area between the frame and the impact plate is also a weak point, easily leading to separation. Overall floating is caused by an uneven bottom working layer during turbulence generator installation, allowing molten steel to seep into the gap between the turbulence generator and the working layer after casting begins, causing the entire turbulence generator to float.

Solutions: First, adjust the turbulence generator formula to control the expansion at high temperatures; second, apply a layer of refractory powder to the working layer surface during installation to ensure a seamless connection between the turbulence generator and the working layer.

2.3 Tunnel Nozzle Cracking and Steel Leakage

Tunnel nozzles containing zirconium cores are prone to cracking during casting, leading to steel leakage, often forcing continuous casting to stop production or causing abnormal shutdowns. Analysis suggests that nozzle cracking is mainly due to the poor thermal shock resistance of the zirconium core.

Solutions: The bulk density of the zirconium core should not be too high, as higher bulk density results in poorer thermal shock resistance; appropriately thickening the nozzle billet can prevent molten steel leakage.

2.4 Ladle Sleeve Fracture

The ladle sleeve is located between the ladle's drain outlet and the tundish. Its metallurgical function is to prevent molten steel from splashing and oxidizing during the flow from the ladle into the tundish. The most common problem with the ladle sleeve during use is fracture. There are two main causes: first, poor thermal shock resistance; second, when replacing the ladle, the sleeve needs to be separated from the drain outlet. If the sleeve is stuck tightly to the drain outlet, external force applied by the casting operator can cause it to fracture.

Solutions: First, use materials with low coefficients of thermal expansion and modulus of elasticity to improve the sleeve's thermal shock resistance. Second, when the sleeve cannot be separated from the drain outlet, do not apply external force to the lower part of the sleeve; instead, apply external force to the upper steel shell of the sleeve to separate it.

3.Conclusion

3.1 Problems encountered during the use of refractory materials in tundishes are sometimes due to the quality of the materials themselves, and sometimes related to on-site construction. Careful observation and analysis are necessary.

3.2 Lack of strength in dry refractory materials is related to poor baking results, moisture absorption, or lack of binder. The floating of the turbulence diffuser is caused by excessive expansion at high temperatures or gaps at the bottom during installation. Cracks and fractures in the nozzles and ladle sleeves are mainly related to the poor thermal shock resistance and high-temperature strength of the materials.