Continuous Casting

Continuous casting is the process by which liquid steel is solidified into a semi-finished solid form — slab, bloom, or billet — for direct feeding to a rolling mill. Before continuous casting was developed industrially in the 1960s and 1970s, steel was cast into individual ingots, which required extensive reheating, soaking, and rolling in a cogging or blooming mill before further processing. Continuous casting eliminated these steps, improving yield by 10–15%, reducing energy consumption by 1–2 GJ/t, and enabling a direct link between the steelmaking and rolling operations.

By 2000, over 90% of global steel production was continuously cast; today the figure exceeds 96%. The transition from ingot to continuous casting is one of the most consequential process innovations in 20th-century steelmaking, comparable in productivity impact to the introduction of the BOF itself.

A continuous caster consists of a ladle turret, a tundish, a water-cooled copper mould, a curved withdrawal and straightening unit, and a secondary cooling zone extending over 10–25 metres. Liquid steel flows from the ladle through a submerged entry nozzle into the tundish, and from the tundish through a second nozzle into the oscillating copper mould where initial solidification occurs. The partially solidified strand is withdrawn downwards (or in a curved path), while secondary water sprays continue cooling until the strand is fully solid. The strand is then torch-cut into slabs, blooms, or billets of the required length.

The tundish is an intermediate vessel — a refractory-lined steel box of 15–60 t capacity — positioned between the ladle above and the mould below. Its primary function is to act as a buffer, maintaining a steady flow of steel into the mould as ladles are exchanged during a casting sequence (a sequence may involve 10–30 ladles, each of 150–350 t). Without the tundish, each ladle exchange would require stopping the caster, generating a ladle-by-ladle sequence of unjoined strands. The tundish allows the ladle to be changed in typically 90–120 seconds with the caster running continuously.

The tundish also acts as a metallurgical reactor. Its volume and dwell time (typically 8–15 minutes at standard flow rates) allow floating of oxide inclusions — Al₂O₃, calcium aluminates, silica — to the tundish slag layer where they are absorbed, preventing them from entering the mould and creating defects in the final product. Tundish design (turbostoppers, dams, weirs, flow modifiers) is optimised for inclusion removal by promoting plug flow (uniform residence time) and minimising short-circuit flow from inlet to outlet.

Tundish temperature control is critical: the steel must arrive at the mould at the correct superheat (temperature above the liquidus) — typically 15–35 °C above the liquidus for slab casting. Too high superheat deepens the liquid core (metallurgical length), increases segregation, and risks mould-related defects. Too low superheat risks partial freezing in the tundish or nozzle, interrupting flow. Plasma tundish heating or induction heating is used at some casters to compensate for heat losses and maintain superheat within ±5 °C during the entire sequence.

The continuous caster mould is a water-cooled copper tube or set of copper plates forming the cross-section of the strand — typically 100–250 mm thick and 900–2,100 mm wide for slab casters; 100–400 mm square for bloom casters; 80–180 mm square for billet casters. Copper is chosen for its exceptional thermal conductivity — the mould must extract approximately 1–3 MW of heat per square metre of mould face during initial solidification, forming a solid shell 10–20 mm thick in the 0.6–1.5 m mould length.

The mould oscillates vertically at 60–300 cycles/min with an amplitude of 3–10 mm. Oscillation serves two functions: it prevents the solidifying shell from sticking to the mould wall (which would cause a longitudinal tear and breakout), and it creates a controlled mould mark pattern on the strand surface (oscillation marks) that acts as a record of mould behaviour. The difference between the mould oscillation speed (downward stroke) and strand withdrawal speed is the "negative strip" period — the downward push that mechanically shears the meniscus and prevents sticking.

Mould flux (also called mould powder or casting powder) is added continuously to the top of the mould — a granulated or powdered blend of CaO, SiO₂, Al₂O₃, Na₂O, and fluorides that melts in the high-temperature region near the meniscus and flows between the mould wall and the solidifying shell. The liquid flux film lubricates the strand-mould interface (preventing sticking) and provides horizontal heat transfer from the shell to the mould wall. Flux composition is tailored for each steel grade: peritectic steels (0.09–0.16% C) require fluxes with controlled viscosity to manage the volume contraction at the peritectic transformation; high-carbon steels, stainless steels, and IF grades each have specific flux requirements.

Mould powder performs five distinct functions, all simultaneously:

1. Lubrication: The molten flux layer between the copper mould wall and the solidifying steel shell prevents direct metal-mould contact, allowing the strand to withdraw without sticking. Flux consumption (typically 0.3–0.6 kg/m² of strand surface) reflects the volume of liquid flux drawn downward by the strand at each oscillation stroke.

2. Heat transfer control: The flux layer is the primary thermal resistance between the steel shell and the copper mould. A higher-viscosity flux creates a thicker, more insulating film — slower heat transfer, better for grades prone to longitudinal cracking. A lower-viscosity flux gives faster heat transfer — required for high-speed casting of thin slabs.

3. Inclusion absorption: Floating inclusions from the steel bath can be absorbed by the molten flux layer at the meniscus if the flux has sufficient absorptive capacity for the inclusion type (typically Al₂O₃ and calcium aluminates).

4. Thermal insulation at the meniscus: The unmelted powder layer on top of the molten flux insulates the meniscus from excessive heat loss, preventing freezing and maintaining a stable liquid pool at the strand surface.

5. Oxidation protection: The powder blanket prevents direct contact between the steel surface and atmospheric oxygen at the meniscus zone.

The oscillation cycle (typically 80–200 strokes/min, ±2–5 mm amplitude) creates oscillation marks on the strand surface at regular intervals (spacing = casting speed / oscillation frequency). These marks are inherent to the process and their depth and shape reveal information about mould flux performance: deep, irregular marks indicate flux instability or excessive friction; smooth, uniform marks indicate good lubrication. The negative-strip portion of each oscillation cycle — when the mould moves downward faster than the strand — is critical: it mechanically detaches the solidifying meniscus from the mould face, preventing the "hook" formation that leads to longitudinal surface cracks in peritectic steel grades.