Mold Region Phenomena
Importance of the Mold Region
The mold region is the most critical zone in the continuous casting machine. Within approximately 1 meter of vertical travel, liquid steel must form a solid shell strong enough to contain the remaining liquid core—typically at pressures of several atmospheres—while simultaneously developing the surface quality that will persist through all downstream processing. Most serious casting defects originate in the mold.
CCC research has focused heavily on mold region phenomena because mathematical modeling provides unique insight into conditions that are extremely difficult to measure directly in an operating caster.
Turbulent Flow in the Mold
Liquid steel enters the mold at high velocity through the submerged entry nozzle (SEN), creating complex three-dimensional turbulent flow patterns. The typical "double-roll" flow pattern directs two jets from the SEN ports toward the narrow faces, then splits into an upper roll (toward the top surface) and a lower roll (toward the bottom of the mold).
Key consequences of mold flow include:
- Surface velocity: Excessive surface velocity causes mold flux entrainment and surface turbulence; insufficient velocity leads to cold meniscus and hook formation.
- Temperature distribution: Flow determines where hot steel impinges on the solidifying shell, affecting local shell thickness and potential for breakout.
- Inclusion transport: The flow pattern governs whether inclusions and argon bubbles are carried to the top surface for removal or captured by the solidifying shell.
Shell Solidification and Growth
The solidifying shell forms at the meniscus (the top free surface in the mold) and grows progressively thicker as it descends. Shell growth is governed by:
- Heat extraction through the mold wall
- Steel grade (liquidus and solidus temperatures, latent heat)
- Casting speed
- Mold flux gap resistance
- Local variations in mold-shell contact pressure
The shell must reach a minimum thickness (~20 mm for slabs) at the mold exit to support the ferrostatic pressure of the liquid core. Local thinning of the shell due to hot spots or excessive heat input can lead to breakouts—catastrophic failures where liquid steel escapes the strand.
Meniscus Behavior and Hook Formation
The steel meniscus—the free surface at the top of the mold—is a critical location for surface quality. During each oscillation cycle, the mold moves downward faster than the strand (negative strip time), causing the meniscus to overflow slightly and form a hook or fold in the solidifying shell. These hooks can trap:
- Mold flux (appearing as inclusions in the final product)
- Argon bubbles (appearing as pinholes)
- Aluminum oxide clusters
Controlling meniscus behavior through oscillation parameters, casting speed, and SEN design is essential for producing defect-free surfaces on exposed-quality steels.
Mold Flux
Mold flux (also called mold powder) is added to the top surface of the steel in the mold. It serves multiple critical functions:
- Thermal insulation: Prevents premature solidification of the meniscus and maintains steel temperature.
- Chemical protection: Absorbs inclusions that float to the surface and prevents reoxidation.
- Lubrication: Liquid flux infiltrates the gap between the mold wall and solidifying shell, reducing friction and preventing sticking.
- Heat transfer control: The crystallization behavior of solidified flux in the gap controls the rate of heat extraction, affecting shell growth uniformity.
CCC has developed extensive models of mold flux behavior, including melting rate, viscosity, crystallization kinetics, and gap heat transfer.
Electromagnetic Effects
Many modern casters employ electromagnetic devices to control flow in the mold region:
- Electromagnetic stirring (EMS): Rotates or oscillates the liquid steel to homogenize temperature and composition, modify flow patterns, and improve solidification structure.
- Electromagnetic braking (EMBr): Applies a static magnetic field to slow the high-velocity SEN jets, reducing surface turbulence and deep penetration of inclusions.
- Electromagnetic level stabilization (EMLS): Damps surface level fluctuations caused by flow asymmetry.
CCC models incorporate magnetohydrodynamic (MHD) coupling to simulate these electromagnetic effects and optimize device parameters.