Continuous Casting

Below the mould, the strand is supported by a series of driven rolls that prevent bulging from the ferrostatic pressure of the liquid core (which can reach 1–2 bar at the strand base). Water and air-mist sprays in the secondary cooling zone extract heat at controlled rates through the roll gaps, driving the solidification front inward until the strand is fully solid at the "metallurgical length" — typically 8–25 m from the meniscus for a slab at standard casting speed.

Secondary cooling is divided into multiple zones, each independently controlled to achieve the target surface temperature profile. A typical slab caster has 4–8 secondary cooling zones. The key criterion is avoiding rapid temperature changes at the strand surface that cause thermal stress and surface cracking — particularly in the temperature range 700–900 °C where austenite is mechanically weak (the "hot brittleness" region). Cooling rates are deliberately reduced ("gentle cooling") as the strand surface passes through this temperature range.

Modern casters use heat transfer models to calculate the solidification front position in real time, adjusting secondary cooling water flows to track the target metallurgical length as casting speed varies. Electromagnetic stirring (EMS) — applying rotating electromagnetic fields in the mould, strand, and final solidification zone — is used on grades requiring reduced centreline segregation: the electromagnetic force circulates the liquid pool, breaking up dendritic bridges and promoting uniform solute distribution. Soft reduction — applying a small mechanical reduction (1–5 mm) to the strand near the final solidification point — further reduces centreline voids and segregation by mechanically compressing the remaining liquid just before it solidifies.

Continuous casting introduces characteristic defects — each with a specific metallurgical origin and a specific set of process controls to suppress it.

Centreline segregation is the enrichment of carbon, manganese, phosphorus, and sulphur at the last-to-solidify centre line of the strand. As dendrites grow inward from the solidification front, solute elements are rejected into the residual liquid, which concentrates progressively. If the strand bulges (from ferrostatic pressure between rolls) or if dendritic bridges form across the liquid pool, solute-enriched liquid is sucked into the centre — creating compositional bands that cause mechanical property gradients between the surface and the core of the rolled product. Control measures: soft reduction (mechanically squeeze out the enriched liquid just before the solidification end), reduce casting speed (shorter metallurgical length, less time for bridging), and electromagnetic stirring (break up dendritic bridges, homogenise solute).

Transverse surface cracks form at oscillation marks in the unbending zone (where the curved strand is straightened to horizontal). Steels in the peritectic carbon range (0.09–0.16% C) are especially susceptible because the δ→γ transformation involves a 0.5% volume contraction that concentrates stress at oscillation marks. Steels containing aluminium at temperatures of 600–900 °C are also susceptible to intergranular embrittlement (AlN precipitation at prior austenite grain boundaries). Control measures: avoid casting in the peritectic carbon range (adjust composition away from 0.12% C), use mould flux with controlled solidification to minimise oscillation mark depth, and design secondary cooling to avoid the unbending zone coinciding with the 700–900 °C surface temperature range.

Breakouts occur when the solid shell below the mould ruptures. Causes: sticking events (shell bonds to mould), overly thin shell from excessive superheat or incorrect mould taper, and oscillation mark stress concentrators. Detected by Breakout Prevention Systems (thermocouple arrays in mould copper plates). Modern BPS detects >95% of potential breakouts before shell failure.

Internal cracks in the secondary cooling zone result from thermal stress when the surface temperature fluctuates — for example, at the transition between secondary cooling zones, or if a spray header blocks and then suddenly unblocks. Cracks form perpendicular to the direction of maximum stress and appear as off-corner cracks or midway cracks in the slab cross-section. Prevention requires smooth, uniform secondary cooling profiles and regular maintenance of spray nozzles.

Electromagnetic stirring applies rotating magnetic fields to the liquid core of the strand, inducing Lorentz forces that drive a rotating flow in the melt. This technology addresses two fundamental limitations of purely diffusion-driven solidification: the tendency to form long columnar dendrites that trap solute and create internal defects, and the tendency for the final solidification centre to develop voids and severe segregation.

Mould EMS (M-EMS): Applied at the mould level, where the steel is still largely liquid. The rotating flow promotes the Columnar-to-Equiaxed Transition (CET) by breaking off dendritic tips from the solidification front, redistributing them as nuclei throughout the liquid pool. The result is a larger equiaxed grain zone at the strand centre, reduced columnar crystal depth, and improved surface quality. M-EMS is standard on billet and bloom casters; it is less commonly used on slab casters because the wide, thin cross-section makes effective field penetration more difficult.

Strand EMS (S-EMS): Applied in the secondary cooling zone, where the strand has a thick shell but a still-liquid core. S-EMS stirs the liquid core longitudinally, breaking up dendritic bridges and equalising solute concentration. The primary benefit is reduced centreline segregation in slab and bloom products. S-EMS is standard on bloom casters for seamless pipe and rail grades, where centreline quality requirements are stringent.

Final EMS (F-EMS): Applied near the solidification end point, where only a small residual liquid pool remains. F-EMS dramatically improves the equiaxed zone fraction in the very centre of the strand — particularly important for bloom and billet products used in high-fatigue applications such as bearings, springs, and rails.

Electromagnetic braking (EMBR): The converse of EMS — instead of driving rotation, EMBR applies a static magnetic field that opposes the high-velocity jet from the SEN. In slab casters, the SEN jet enters the mould at 1–3 m/s and penetrates deep into the liquid pool, potentially carrying inclusions past the solidification front into the interior of the slab. EMBR slows the jet, reducing penetration depth and allowing inclusions more time to float before they become entrapped by the advancing solidification front. EMBR is particularly important for ultra-clean automotive and electrical steel grades where surface and sub-surface inclusion count must be minimised.