Process Overview
The Casting System
The continuous casting system comprises several interconnected components, each playing a critical role in producing high-quality steel strands. Understanding the interactions among these components is essential for optimizing the process and preventing defects.
Ladle Turret and Ladle
The casting sequence begins at the ladle turret, where ladles of refined liquid steel are positioned above the tundish. A ladle typically holds 150–350 tonnes of liquid steel at approximately 1550–1580°C. The ladle slide gate or stopper rod controls the flow of steel into the tundish. The turret allows one ladle to be casting while the next is prepared, enabling continuous operation across ladle changes.
Tundish
The tundish is a refractory-lined vessel that holds 15–50 tonnes of liquid steel between the ladle and the mold. Its functions include:
- Buffering steel flow to enable ladle changes without interrupting casting
- Distributing steel to multiple molds in multi-strand machines
- Providing residence time for inclusion flotation and removal
- Maintaining thermal and chemical homogeneity
- Protecting steel from reoxidation and temperature loss
Tundish design (shape, dam/weir configurations, flow control devices) significantly affects steel cleanliness and product quality.
Submerged Entry Nozzle (SEN)
The submerged entry nozzle (SEN) channels steel from the tundish into the mold. It is submerged below the mold flux/steel interface to prevent reoxidation and minimize surface turbulence. The SEN design (bore diameter, port angle, port area) strongly influences the turbulent flow pattern in the mold, affecting:
- Mold flux entrainment and surface level fluctuations
- Inclusion and argon bubble distribution
- Shell growth uniformity
- Temperature distribution in the mold
Argon gas is typically injected through the SEN to prevent clogging by alumina inclusions and to modify flow patterns.
Mold
The oscillating, water-cooled copper mold is the heart of the continuous casting process. It provides primary cooling to form the initial solid shell that contains the liquid steel core. Key mold parameters include:
- Mold geometry: width, thickness, taper, and curvature
- Mold material: copper alloy grade and coating
- Cooling water: flow rate, velocity, and temperature
- Oscillation: frequency (typically 1–4 Hz) and stroke (typically 3–10 mm)
- Mold flux: physical and chemical properties
Mold oscillation creates a negative strip time during which the mold moves downward faster than the strand, helping to detach the shell from the mold wall and distributing mold flux into the gap between mold and shell.
Secondary Cooling Zone
Below the mold, the partially solidified strand passes through the secondary cooling zone, where water sprays and radiation cooling complete solidification. This zone typically extends 10–20 meters below the mold. The secondary cooling practice must balance:
- Rapid cooling to maintain productivity and shell strength
- Uniform cooling to minimize thermal stresses and surface cracking
- Controlled cooling at critical transformation temperatures
- Avoidance of overcooling that causes thermal shock and transverse cracking
Withdrawal and Straightening
Drive rolls withdraw the strand from the mold at the casting speed. In curved-mold machines (the most common configuration), the strand must be straightened from its curved path to a horizontal direction before cutting. Straightening applies mechanical strains to the partially solidified strand, and if done at the wrong temperature or with excessive speed, can cause internal cracking. Mathematical models of shell growth and stress are essential for optimizing the straightening process.