Continuous Casting Tundish Lining Refractories: Practices, Advantages and Disadvantages

Tundish (Background)

Continuous casting represents a significant advancement in steelmaking technology. The refractories used in continuous casting play a critical role in controlling the molten steel during the final stage of processing, requiring them to possess high stability and specialized properties. In any continuous casting operation, the tundish functions as a buffer vessel between the steel ladle and the mold. It serves as both a reservoir and fulfills several metallurgical functions, such as facilitating the flotation of inclusions, regulating flow to the molds, and ensuring thermal and chemical homogenization.

Over the years, there has been an ongoing evolution in the practices surrounding tundish refractory linings globally. Transitioning from a simple reservoir and distribution vessel, the tundish is now recognized as a steel refining vessel, leading to the emergence of a new domain in steelmaking technology known as Tundish Technology.

Types of Tundish Lining Refractories - Advantages and Disadvantages

There is a host of different tundish lining refractories which can be categorized into 5 major types (also in a roughly chronological order):     

Brick Lining

Gunnable Tundish Lining

Tundish Board Lining

Sprayable Tundish Lining

Tundish Dry Lining (in-situ formed)

Brick Lining  

The introduction of continuous casting in the 1960s initially relied on the same refractory brick lining technology used in other metal-containing vessels. High-alumina refractories were employed for tundish linings, which came into direct contact with liquid steel after pre-heating. This method was essentially an extension of ladle refractory practices to the tundish and continuous casting process.  

Merits:  

- Low risk of hydrogen pickup by molten steel  

- No sand required  

- Lower inventory needs  

- No investment in specialized equipment  

- Reduced washout risk  

Demerits:  

- Intensive curing is required  

- Highly labor-intensive  

- Poor insulation properties  

- Late-stage temperature drops during casting due to the high thermal conductivity of the brick lining, resulting in heat loss that can affect metallurgical parameters  

- "Cold start" operations are not possible  

- A large tundish fleet is necessary  

- Difficulties in deskulling (stripping)  

- Issues with joints  

- Long preparation time for tundishes  

The numerous challenges associated with brick linings led some operators to opt for trowelable and, subsequently, gunnable linings, despite the added costs.

Gunnable Tundish Lining  

The commercial use of gunnable refractory lining in tundishes began in Japan as a solution to some of the challenges associated with bricked linings. Initially, these tundish lining refractories were alumino-silicate-based but later transitioned to magnesite-based or basic types to better align with metallurgical practices. In this method, dry refractory powder of the appropriate composition is fluidized and then applied to the tundish walls using a gunning machine, creating a monolithic lining. Although this approach resulted in a joint-free structure and improved deskulling, it did not significantly reduce preheat times or heat losses due to the relatively high density of the gunned linings. Furthermore, there remained a tendency for the linings to crack and spall during rapid preheating, which prevented the use of gunnable refractory lining for cold start operations.  

Merits:  

- Low risk of hydrogen pickup by molten steel  

- No sand required  

- Lower inventory needs  

- Joint-free structure  

- Less labor-intensive  

- Relatively easy installation with shorter timeframes  

- Easier deskulling compared to brick linings  

Demerits:  

- Intensive curing required  

- High material wastage due to rebound losses  

- Poor insulation properties  

- "Cold start" operations are not possible  

- High risk of washout  

- Low thermal stability  

- High shrinkage can lead to stress and crack formation during operation, while low shrinkage can complicate deskulling  

- Dust generation issues  

- Energy-intensive processes  

- Long turnaround cycles  

- Higher costs  

- Required investment in equipment  

Tundish Board Lining  

In the mid-1970s, a new type of tundish wear lining was introduced: board systems made from low-density, highly insulating, disposable, pre-formed, and pre-cured refractory boards. These systems gained immense popularity among steelmakers due to their ease of deskulling, low equipment investment, and cost-effectiveness, particularly with silica varieties. Initially, silica-based boards were used, allowing for "cold start" practices. However, magnesite-based boards were introduced in the mid-1980s to meet the pre-heatability requirements necessary for "hot start" operations, particularly for producing high-alloy quality steels with low hydrogen content. Despite their advantages, the board system still faced challenges such as labor intensiveness, the presence of joints, sand backing, and breakages.

Merits:  

- Low risk of hydrogen pickup (when hot)  

- Uniform liner shape  

- No curing required  

- Good insulation properties  

- Cold start operations possible  

- Easy deskulling  

- Low energy requirements  

- Short turnaround cycle  

- No investment in equipment  

- Low washout risk  

- Cost-effective (for silica-based boards)  

Demerits:  

- Presence of joints  

- Sand backing required  

- Risk of hydrogen pickup (when cold)  

- Labor-intensive application  

- High inventory needs  

- Handling and breakage issues  

- Higher costs (for magnesite-based boards)  

In response to the growing demand for cleaner steel and advancements in continuous casting technology, a variety of refractories for tundish lining have emerged in the market. Nevertheless, board systems remain popular in countries with low labor costs where application technologies are less accessible. Tundish boards, especially those developed for energy savings, are increasingly being accepted across the industry.

Tundish Dry Lining  

Dry tundish linings were introduced in Europe around 1986, representing a significant shift from previous methods by utilizing a dry powder application that does not require water for the tundish working lining. This system typically employs a resinous bond activated by relatively low heat (approximately 160°C). Vibration may or may not be necessary, depending on the specific product, but the use of a former is essential. The dry powder is fed into the space between the tundish's permanent lining and the former. Hot air is then introduced at about 400°C, with the heating cycle lasting around 45 minutes, followed by an additional 30 minutes for cooling.  

This method saves time; however, it has some drawbacks, including lower insulation due to higher density and a reliance on crane systems for installation in the tundish bay. A significant advantage of dry lining is the absence of water in the process, which prevents direct adhesion to the permanent tundish lining. This feature enhances deskulling and extends the lifespan of the tundish lining. Additionally, the smooth finish of a dry tundish lining and the ability to consistently reproduce the lining geometry improve steel quality and erosion resistance, potentially increasing casting sequence lengths.  

Merits:  

- No joints  

- No sand required  

- Low hydrogen risk (when hot)  

- Low inventory needs  

- Less labor-intensive  

- Reduced tundish preparation time  

- Low washout risk  

- Easy deskulling  

- Uniform lining  

- Environmentally friendly application  

- High sequence lengths achievable  

- Benefits in OH and S emissions  

- Easy and quick installation  

- Improved steel cleanliness due to lining integrity  

Demerits:  

- Equipment investment required  

- Hydrogen risk (when cold)  

- Lower insulation properties  

- Dependence on cranage for installation  

The demand for higher performance refractories in continuous casting is increasing, driven by the challenges posed by molten steel, which subjects these materials to severe corrosion and abrasion. The choice of refractories significantly impacts steel quality and yield points. While bricked and gunned systems offer certain advantages, their drawbacks often outweigh their benefits. Conversely, despite some disadvantages associated with board, spray, and dry lining systems, the advantages appear to be more substantial. Therefore, selecting between these options is complex, as the merits and demerits are nearly balanced. It is crucial to consider additional factors, such as steel plant operations and steel quality, when deciding among board, spray, and dry linings.