Tundish Dry Vibratable Mix
The tundish working lining is in direct contact with molten steel, requiring it to reduce inclusions in the molten steel, improve steel cleanliness, and exhibit excellent resistance to erosion by molten steel and slag. This ensures the longest possible casting duration for thin-slab continuous casting while allowing for easy dismantling after use to reduce labor intensity.
In recent years, the development trends of tundish working linings have been reflected in two aspects:
1)Form evolution: from insulation boards to spray coatings and dry vibratable mixes;
2)Material advancement: the development of magnesium-based, magnesia-calcium-based, and other compositions to achieve molten steel purification.
01 Process Requirements
The tundish dry vibratable mix uses a composite binder composed of organic and inorganic binders. Since the organic binders decompose at a certain temperature and lose their bonding effect, the temperature of the permanent lining during construction must not be too high. In response to the plant's continuous casting requirements, specific requirements for the thickness of the working lining and other parameters are proposed, as shown in Table 1.
02 Construction Process
The tundish dry vibratable mix adopts a construction process of vibration molding and heat curing. Construction can be carried out when the temperature of the permanent lining meets the construction requirements.
Step-by-Step Construction Procedures:
1)First, install the lower impact plate and seating bricks, and ram them tightly with ramming mass. Then pour the dry vibratable mix onto the tundish bottom, ensuring the required thickness and flatness.
2)After completing the tundish bottom construction, install the tundish formwork and adjust the distance between the formwork and the permanent lining.
3)Pour the dry vibratable mix into the tundish until it reaches the upper edge. Start the vibration motor for 3–5 minutes, then add more mix and repeat the vibration process until the mix is fully compacted and reaches the tundish edge.
4.During construction, ensure that no foreign materials (such as packaging bags, plastic liners, etc.) are introduced into the tundish.
Curing and Demolding:
1)Ignite the baking burners on the formwork for low-temperature curing. The curing temperature should be maintained at approximately 200–250 °C for 1–1.5 hours to ensure that the powder in contact with the formwork hardens and develops sufficient strength.
2)Turn on the cooling fan for air cooling. Demolding can be performed when the temperature of the formwork drops below 100 °C. Care must be taken during demolding to avoid damaging the constructed dry vibratable mix lining.
Post-Demolding Inspection and Installation:
1)After demolding, inspect the working lining surface for defects such as spalling, collapse, or cracks.
2)Install precast components (e.g., impact pads, weirs) and submerged entry nozzles (SEN) with precise alignment and secure fixation.
3)After installing the precast components, evenly pour dry vibratable mix around the impact pads and weirs to fill the gaps.
4)Once the entire construction is completed, install the stopper rod, and the tundish is ready for on-site use.
03 Baking Curve
Since the tundish working lining uses a dry vibratable mix (which contains no moisture), it generally does not develop cracks or collapse during high-temperature baking. For pre-casting baking:
• Control the temperature within 800 °C for the first 30 minutes;
• Subsequently, perform high-temperature baking to raise the temperature to 1100 °C;
• The minimum baking duration is 70 minutes, and the maximum baking time should be controlled within 180 minutes.
Because the tundish dry vibratable mix uses organic binders, prolonged baking will impair the binder performance, accelerate carbon oxidation in the mix, and reduce the erosion resistance of the working lining.
Advantages of Tundish Dry Vibratable Mix
The tundish working lining made of spray coating has the following drawbacks in practical application:
1)Highly sensitive to environmental conditions. In winter, measures must be taken to remove moisture from the working lining to prevent freezing;
2)Poor construction conditions and high labor intensity;
3)Crack formation after natural curing or pre-baking;
4)Collapse or spalling of the working lining during baking or casting;
5)Unstable resistance to erosion by molten steel and slag;
6)Severe sintering during long-duration continuous casting, resulting in failure to achieve automatic tundish stripping.
Compared with spray coatings, tundish dry vibratable mix offers the following advantages:
1)Excellent resistance to erosion by molten steel and slag, enabling long-duration casting and reducing the refractory cost per ton of steel;
2)Low tendency to develop cracks or collapse during baking;
3)Ready for on-site use immediately after construction, accelerating tundish turnover and facilitating production organization;
4)Easy disintegration of the residual lining and convenient tundish stripping;
5)The working lining sinters layer by layer during casting, with powdery material remaining inside. This minimizes slag adhesion on the permanent lining and contributes to an extended service life of the permanent lining.
04 Service Performance
To date, a total of 60 tundishes equipped with dry vibratable mix working linings have been put into operation, achieving a maximum of 12 consecutive heats of continuous casting. Post-operational inspection of the retired linings reveals the following performance metrics:
• Erosion rate: 1–2 mm·h⁻¹;
• A sintered layer of approximately 15 mm is formed, with the powdery core remaining intact;
• Tundish stripping is straightforward, with negligible material adhesion, ensuring no damage to the permanent lining.
Detailed service data are provided in Table 2.
Table 3 presents a comparison of service performance between the dry vibratable mix working lining and the spray coating working lining, showing that the former exhibits superior erosion resistance compared with the latter.
For casting heats using the dry vibratable mix working lining, comparative analysis of slab compositions confirms that the working lining does not contaminate the molten steel, and no inclusions introduced by the working lining have been detected.
05 Issues Encountered During Application
1. Entire Constructed Lining Removed Along with Formwork During Demolding
When demolding after completing the tundish wall construction, the entire dry vibratable mix lining was inadvertently removed together with the formwork. Analysis indicates that the problem may be associated with the following factors:
1)Residual material on the formwork was not thoroughly cleaned before use;
2)The lubricant applied to the formwork surface was uneven, failing to achieve effective lubrication.
In subsequent construction, targeted measures were implemented: the formwork is now thoroughly cleaned immediately after construction, and the viscosity of the lubricant is adjusted to ensure optimal adhesion during application. No recurrence of this issue has been reported since.
2. Large Cracks on the Constructed Lining Surface After Demolding
During the application of the dry vibratable mix, large cracks appeared on the constructed lining surface after demolding on two occasions. The cracks were generally longitudinal, located on the tundish wall in the impact zone, extending from the tundish edge to the bottom, with depths reaching the powdery layer, though they did not affect service performance.
Analysis suggests that the issue may be related to the following factors:
1)Prolonged baking led to an excessively thick sintered layer, while insufficient cooling time resulted in relatively high surface temperatures of the tundish wall during demolding;
2)After demolding, the constructed lining was directly exposed to room temperature. Shrinkage stress was generated during the cooling process of the tundish wall, and when this stress exceeded the material's intrinsic strength, cracks occurred.
To address these problems, the optimal baking temperature, baking duration, and air cooling time were further adjusted. Additionally, appropriate toughening agents and low-temperature expanders were added, effectively controlling the initiation and propagation of cracks.
3. Severe Erosion at the Slag Line
Currently, the steel grade cast at the plant is medium carbon steel, with alkaline covering agents used during the process. Post-service inspections have revealed that the working lining at the slag line occasionally suffers from severe erosion, with an erosion rate exceeding 4 mm·h⁻¹. Analysis indicates that the potential causes are as follows:
1)Insufficient erosion resistance of the dry vibratable mix. To address this issue, the mix formulation has been adjusted to enhance its erosion resistance. Practical application has confirmed that the optimized dry vibratable mix exhibits superior erosion resistance at the slag line compared with the original version.
2)Occasionally, due to various factors, the tundish preheating time before casting is excessively prolonged, leading to oxidation of the surface layer and reduced erosion resistance.
3)Unstable molten steel composition, excessive slag carry-over from the ladle, and inadequate slag removal from the tundish collectively contribute to accelerated erosion of the working lining.
4)Poor stability of the tundish covering agent performance.