Continuous Casting

The continuous caster produces semi-finished shapes that feed directly into downstream rolling or forging operations. The three main formats are:

Slabs (150–350 mm thick × 900–2,100 mm wide) are the feedstock for hot strip mills, plate mills, and pipe mills. A modern high-throughput slab caster operates at 1.2–2.0 m/min casting speed with two or four strands in parallel, producing 2–5 Mt/year per caster. Thin slab casters (45–90 mm thick) cast directly into compact strip production (CSP) lines, where the slab is hot-rolled in-line without a separate reheating furnace — a major capital and energy saving.

Blooms (150–500 mm square or equivalent) are the feedstock for large section rolling mills (heavy sections, rails, large-diameter seamless pipe). Bloom casters often incorporate electromagnetic stirring and soft reduction to achieve the internal quality requirements for these demanding applications.

Billets (80–180 mm square) are the feedstock for long product rolling mills (rebar, wire rod, small sections). High-speed billet casters operate at 4–8 m/min, producing 0.5–1.5 Mt/year per strand, and may incorporate mould electromagnetic stirring to control billet internal quality. Near-net-shape beam blank and round casters are specialised variants for section and seamless pipe production respectively.

Casting speed — the rate at which the strand is withdrawn from the mould — is the primary productivity variable of the continuous caster. Typical casting speeds are 0.8–2.5 m/min for slab casters, 1.0–3.5 m/min for bloom casters, and 3–8 m/min for billet casters. Higher speed increases throughput but shortens the metallurgical length (relative to the strand cross-section), requiring more intense secondary cooling to achieve full solidification before the torch-cutter.

The relationship between casting speed and metallurgical quality is a fundamental trade-off: - Higher speed increases throughput, but increases the ferrostatic pressure at the bottom of the liquid pool (more time for bulging), increases the risk of centreline segregation (longer liquid pool, more time for solute enrichment), and increases secondary cooling demands. - Lower speed reduces throughput but gives more time for inclusion flotation in the tundish, reduces strand bulging, and allows a gentler secondary cooling profile that minimises thermal stresses.

For most slab casters, the optimum casting speed is 1.2–1.6 m/min — a compromise that achieves acceptable productivity while maintaining internal quality. Ultra-clean grades (bearing steel, IF automotive, heavy plate) are typically cast at the lower end or below this range.

Sequence length — the number of successive ladle heats cast without changing the tundish — is a major determinant of caster yield and operating cost. At each ladle change, a transition zone of approximately 5–15 t of steel passes through the tundish as the new heat mixes with the old. For single-grade campaigns (all heats of the same composition), this transition material can be identified and either reclassified to a lower-grade product or cropped. For mixed-grade sequences, the transition material between incompatible grades must be cropped and recycled. Modern plants achieve sequence lengths of 10–30 ladles for same-grade campaigns, dramatically reducing transition losses. The critical enabler is tundish lining durability — modern isostatically pressed refractory tundish linings last 12–20 hours of casting time at elevated temperatures, supporting sequences of this length. The ladle-to-tundish shroud (the SEN connecting the ladle and tundish) is sealed with argon gas to prevent atmospheric reoxidation of the steel stream — the most important reoxidation prevention point in the entire caster.