Intermediate Ladle: What It Does And Its Lining Structure Explained

The intermediate ladle is used as a transitional device for molten steel between the ladle and the crystalliser. It plays the role of "controlling the start and stop" of molten steel flow during continuous casting after it flows from the ladle. Its installation position is shown in Figure 1.

Figure 1: Installation position of the intermediate ladle

1 – Steel ladle; 2 – Intermediate ladle; 3 – Mould; 4 – Second cooling zone

The main tasks of the intermediate ladle are:

Steel distribution – In a multi-stream continuous casting machine, the intermediate ladle distributes molten steel to each mould.

Flow stabilization – It reduces the static pressure of molten steel, maintains a stable molten steel level in the intermediate ladle, and ensures smooth injection into the mould.

Molten steel storage – During ladle changes in multi-furnace continuous casting, it enables continuous casting without reducing the withdrawal speed, thus facilitating multi-furnace continuous pouring.

Steel purification – During longer casting periods, it helps maintain a stable steel temperature, promotes the further flotation of inclusions, prevents contact between steel and air, and avoids absorption of oxygen and nitrogen.

The intermediate ladle is generally composed of a ladle body, ladle cover, water outlet, and stopper rod. The shell of the ladle body is a metal structure, and the inner lining is made of refractory material. It is generally rectangular, but can also be T-shaped, trapezoidal, or V-shaped. These shapes are mainly designed to reduce the eddy currents generated by the molten steel during injection, and also to facilitate slag removal and hanging during operation.

The shell of the intermediate ladle is welded from steel plates and must have sufficient rigidity to withstand high temperatures without deformation during handling and cleaning. For this reason, reinforcement plates are welded to the exterior of the shell. In large-capacity intermediate ladles, a partition wall is included to isolate the steel flow from the ladle, which reduces turbulence in the molten steel inside the intermediate ladle. To minimize heat loss, the intermediate ladle should also be covered with insulation.

The key structural parameters of the intermediate ladle include its length, width, and capacity. The length mainly depends on the number of casting strands and the spacing between them. The distance from the water outlet to the end wall of the ladle is generally no less than 200 mm. With these two parameters, the length of the ladle can be determined. The width is primarily based on the position of steel injection and the distance to the water outlet. This distance should promote effective steel flow distribution and prevent dead zones. The distance from the point of molten steel impact to the nearest water outlet should not be less than 500 mm.

If the ladle body is too wide, it increases the heat dissipation area and reduces thermal insulation performance. It also increases the weight of the ladle, the size of the ladle carriage, and complicates the layout of supporting equipment. The capacity of the intermediate ladle is generally 20% to 40% of the ladle's capacity. In recent years, there has been a trend toward increasing this percentage. During multi-furnace casting, the intermediate ladle must be able to store enough molten steel to ensure continuous casting for at least 5 minutes during ladle changes.

According to the role of the intermediate ladle, its structure should meet the following requirements:

Minimize heat dissipation area

Provide good thermal insulation

Have a simple shape for easy bricklaying, cleaning, and pouring operations

The arrangement of water outlets should match the billet cross-section and number of strands

Withstand long-term exposure to high temperatures

Be structurally stable and reliable

The commonly used shape and size of the intermediate ladle are determined by the number and position of casting strands. Multi-strand continuous casting machines usually use long, narrow intermediate ladles, while rectangular ladles are typically used for single-strand casting machines.

The capacity of the ladle is determined by the continuous casting speed and should generally be slightly larger to allow for continuous casting during ladle changes. Additionally, there must be sufficient static pressure head to ensure stable steel outflow, reduce turbulence, and promote the flotation of non-metallic inclusions. Therefore, the walls and bottom of the ladle are inclined, and sometimes slag walls or partition walls are added.

A schematic diagram of the intermediate ladle structure is shown in Figure 2.

Figure 2: Schematic Diagram of Intermediate Ladle Construction

a – Double-flow long type; b – Long water spout

1 – Ladle cover; 2 – Overflow nozzle; 3 – Ladle wall; 4 – Ladle bottom;

5 – Water spout; 6 – Long water spout; 7 – Immersion nozzle; 8 – Steel surface covering

The lining of the intermediate ladle is made of refractory bricks. The inner wall has a certain taper to facilitate slag removal. Asbestos sheets are placed between the brick lining and the ladle shell to reduce heat dissipation.

The bottom of the ladle is equipped with one or more water ports, while the top is fitted with a cover for heat preservation during casting. This helps prevent deformation of the ladle bottom due to overheating and thermal stress. Before casting, the ladle must be cleaned, and the lining should be preheated to approximately 1100 °C to prevent freezing at the water port when pouring begins.

The proper operation of the intermediate ladle is crucial for ensuring the smooth progress of continuous casting. The following points must be observed during operation:

1.The water port of the intermediate ladle is smaller than that of the main ladle, making it more susceptible to cooling and blockage.

2.The molten steel level in the intermediate ladle is used to regulate and control the steel flow into the crystalliser. A higher molten steel level results in greater outflow; conversely, a lower level reduces flow.

3.The height of the molten steel level should typically be around 550 mm, and must not be less than 300 mm.

4.Pouring temperature (i.e. the temperature in the ladle) varies depending on the steel grade. The ideal pouring temperature is approximately the steel's freezing point plus 30–40 °C.

For example, if the freezing point of a given steel grade is 1510 °C, the appropriate intermediate ladle temperature should be around 1540–1550 °C.

To maintain this range, the molten steel level may need to be adjusted during operation.

5.The temperature difference among outlets in a multi-stream casting machine is typically about 5–10 °C.

In a 3-stream casting machine, for instance:

The middle (2nd) stream tends to be hotter, increasing the risk of breakout due to high fluidity.

The 1st and 3rd streams tend to be cooler, which increases the likelihood of nozzle blockage.

6.The change in steel temperature within the intermediate ladle over time is illustrated in Figure 3.

Figure 3: Temperature Variation of Molten Steel in the Intermediate Ladle

The temperature of molten steel along the height of the ladle wall determines the pattern of temperature change in the intermediate ladle during continuous casting.

Generally, within 15–20 minutes after the start of pouring, the temperature of the intermediate ladle lining gradually increases, and the steel temperature becomes higher than during the initial stage. In the middle stage of pouring, as long as a stable molten steel level is maintained in the intermediate ladle, the surface temperature of the molten steel usually remains relatively stable, with little fluctuation.

In the later stage of pouring, the temperature of molten steel in the intermediate ladle decreases slightly. However, since the lining of the intermediate ladle has already been heated, the rate of temperature drop is less than in the earlier stage.

The preheating level of the intermediate ladle before continuous casting is a major factor affecting the steel temperature drop in the early stage of pouring. For every 300–500 °C increase in the working surface temperature of the ladle lining, the heat loss of the molten steel due to contact with the lining can be reduced by 20%–25%.

Figure 4 shows the relationship between the preheating temperature of the intermediate ladle and the molten steel temperature.

Figure 4: Relationship Between Lining Temperature and Molten Steel Temperature in the Intermediate Ladle

Based on the steel casting and smelting methods, baking requirements, and other factors, intermediate ladles are divided into the following types:

High-temperature intermediate ladle: Used for specific metallurgical processes, lined with magnesium bricks, and able to withstand temperatures up to about 1500 °C.

Hot intermediate ladle: The most common type, lined with burnt or unburnt bricks or castables, and preheated to 800–1100 °C before casting.

Cold intermediate ladle: Lined with insulating plates and can be used without preheating before pouring.

The refractory composition of the intermediate ladle lining consists of the parts shown in Figure 5.

Figure 5: Composition of Refractory Materials for Intermediate Ladles

1 – Ladle shell; 2 – Permanent layer; 3 – Working layer; 4 – Ladle bottom; 5 – Ladle cover;

6 – Stopper rod; 7 – Sleeve brick; 8 – Stopper head; 9 – Seat brick; 10 – Water outlet brick

1.Insulation layer (10–30 mm): This layer is adjacent to the ladle steel shell and usually consists of asbestos sheets, insulating bricks, or lightweight castables. The best results are achieved using insulation fibreboard about 12 mm thick, which has low thermal conductivity and is also easy to install.

2.Permanent layer (100–200 mm): This layer is in contact with the insulation layer. The material is generally clay bricks. This permanent lining is the most common, and the casting material is generally high alumina.

3.Working layer (20–50 mm): This layer is in contact with molten steel and is the key part of the lining. The materials used include semi-siliceous, waxy, clay bricks, high alumina bricks, alkaline bricks (such as magnesium and zirconia bricks). Insulation panels such as siliceous insulation panels, magnesian insulation panels, or magnesian olivine insulation panels may also be used. Additionally, coatings like magnesium, magnesium-chromium, and magnesium-calcium paints are sometimes applied. These materials are commonly used for the intermediate ladle lining.

4.Seat brick: Located at the bottom center of the ladle, this supports the water outlet. The material is usually high alumina.

5.Ladle bottom: The material is generally similar to that of the working layer. The bottom of the intermediate ladle is most susceptible to damage from steel impact and requires good erosion and wear resistance. Common materials include high alumina bricks, dense high alumina bricks, and aluminium-chromium bricks. Special large pieces of high-strength magnesium tar bricks are also used. Zircon bricks may be added in areas subject to heavy steel impact for reinforcement.

6.Ladle cover: Covers the intermediate ladle and serves as insulation while preventing molten steel from splashing. The material is usually clay or high alumina refractory castables.

7.Slag retaining wall (weir): Built inside the intermediate ladle, it can be a single or double wall. The slag retaining wall is usually made of high alumina bricks or prefabricated blocks and is used to retain slag. To improve molten steel cleanliness, a molten steel filter may also be installed on the slag retaining wall.

The continuous casting intermediate ladle was originally used for steel insulation, with its main purpose being steel distribution and flow stabilization. Along with advances in molten steel slow cooling technology, reheating technology, argon gas sealing technology, gas stirring, and the removal of non-metallic inclusions, the continuous casting intermediate ladle has become the most important refining vessel (or furnace) in the final stage of steelmaking, and has been developed on a large scale. Currently, it can achieve steel temperature control, precise adjustment of trace alloying elements, and improve inclusion modification through calcium treatment-this is also known as intermediate ladle metallurgy.

With increasing demands for steel quality, various ladle refining technologies have been continuously developed with the aim of improving steel purity and producing "clean" molten steel. Located between the continuous casting ladle and the crystallizer, the intermediate ladle receives molten steel already refined outside the furnace and further purifies it.

A. Purification Function

To produce high-purity clean steel, porous gas-permeable bricks are installed at the bottom of the intermediate ladle, or porous tubular gas distributors are embedded between the working layer and the permanent layer, or porous bricks are installed on the slag retaining wall for argon blowing. This allows argon bubbles to rise evenly from the bottom, promoting the removal of inclusions. These devices help eliminate hydrogen increase in steel at the start of pouring, reduce oxide inclusions, mitigate defects caused by Al₂O₃ inclusions in tin-plated steel, and significantly reduce the content of non-metallic inclusions.

B. Temperature Regulation Function

To ensure the temperature difference before, during, and after pouring in the intermediate ladle is less than 5 °C and close to the liquidus temperature, thus expanding the equiaxed crystal zone in the cast billet and reducing centerline segregation, measures such as adding small pieces of scrap, spraying powdered iron, and others can be taken in the intermediate ladle to regulate the molten steel temperature.

C. Composition Fine-Tuning

Aluminum, titanium, boron, and other cored wires are fed into the crystallizer through the central hole of the intermediate ladle stopper rod to achieve micro-alloy composition fine-tuning in the steel. This not only improves the yield of oxidation-prone elements but also prevents clogging of the water outlet.

D. Refining Function

A double-layer slag is added to the surface of the molten steel in the intermediate ladle to absorb inclusions floating in the steel. Alternatively, calcium wire can be fed into the intermediate ladle to modify the form of Al₂O₃ inclusions, preventing clogging of the water outlet.

E. Heating Function

Induction heating and plasma heating are used in the intermediate ladle to accurately control the pouring temperature of molten steel within ±3 to ±5 °C. At the start of pouring, during ladle changes, or at the end of pouring, heating is applied to compensate for temperature drops, maintaining the molten steel temperature near the target level. This stabilizes the pouring operation, improves billet quality, and compensates for the natural temperature drop during normal pouring.

Common heating methods include induction heating and plasma heating:

Induction Heating Method

An induction heater is installed at the bottom of the intermediate ladle, heating the steel and providing electromagnetic stirring to promote inclusion flotation. For example, at Japan's Chiba plant, an 8-ton intermediate ladle uses induction heating to control steel temperature within 0 to 5 °C of the target (no heating for 10 to 20 °C). This reduces surface inclusions in cast billets by 25–50%, significantly lowering defects in cold-rolled stainless steel plates to the level of finished products in normal pouring conditions.

Plasma Heating Method

A special arc emitter generates high-temperature plasma to heat the copper water jacket of the ladle. Advantages include:

The plasma generator can be installed above the intermediate ladle, making it easy to operate and adjust.

During heating, molten steel temperature, liquid level, and casting speed are monitored to regulate power input, ensuring precise temperature control.

The method is environmentally friendly and can maintain molten steel temperature within ±1 to ±5 °C of the target.

The ladle can be reused up to 200 times in the hot state, reducing refractory consumption and increasing production efficiency.

Plasma heating also promotes inclusion flotation and helps maintain a complete liquid slag cover layer on the steel surface, preventing secondary oxidation and improving steel purity.

Plasma heating equipment can be installed on intermediate ladles used for both slab and billet continuous casting machines.

F. Large-Scale Development of the Intermediate Ladle

Currently, the capacity of intermediate ladles used in slab continuous casting machines has been gradually increasing. In the 1960s, the capacity was around 6 tons, while by the 1980s most ladles had capacities exceeding 45 tons. At present, the largest intermediate ladle capacity is 80 tons. The advantages of increasing the intermediate ladle capacity are as follows:

1.It prolongs the residence time of steel in the ladle, which favors the flotation of inclusions.

2.It prevents a reduction in casting speed when changing ladles, maintaining stable pouring from the intermediate ladle, and preventing the molten steel level from dropping below the critical vortex value, which would otherwise draw slag into the crystallizer.

3.The steel level remains stable throughout the entire pouring process, which facilitates smooth operation.

4.Increased intermediate ladle capacity helps reduce metal loss and lowers operating costs. Larger intermediate ladles are more conducive to producing clean steel with good surface quality and internal consistency.