Process Overview
In continuous casting, liquid steel flows from the ladle through a shrouded nozzle into an intermediate vessel called the tundish, which buffers temperature and flow between the ladle exchange cycle and the mould. Steel exits the tundish through submerged entry nozzles (SENs) into a water-cooled oscillating copper mould where a solid shell forms around a liquid core. The partially solidified strand is then withdrawn downward (or in a curved machine, downward and then curved to horizontal) through a series of secondary cooling spray zones. Driven by withdrawal rolls, the strand is straightened, torch-cut to length, and discharged as slabs, blooms, billets, or beam blanks. Continuous casting replaced ingot casting across the industry between 1960 and 2000, improving metallic yield from approximately 75% to over 97% and eliminating the soaking pit and primary rolling steps required to break down ingots.
Tundish Metallurgy
The tundish is not merely a distribution vessel: it plays an active metallurgical role. Its volume (15–60 t) provides a residence time of 5–12 minutes during which non-metallic inclusions — alumina, silica, calcium aluminate — float to the surface and are absorbed by the tundish powder or slag. Flow modifiers (dams, weirs, turbostop impact pads) control the steel flow pattern to maximise inclusion flotation and suppress surface turbulence. Tundish temperature must be controlled to within ±5 °C of target superheat (typically 15–35 °C above the liquidus temperature) — too high causes refractory erosion and coarse solidification structure; too low causes freezing at the SEN, blocking the nozzle and potentially causing a breakout.
Mould and Primary Solidification
The copper mould is typically 600–900 mm tall and is internally water-cooled, extracting 1–2 MW of heat per strand. It oscillates vertically at 60–300 cycles per minute with a stroke of 3–10 mm, using a sinusoidal or non-sinusoidal profile. Oscillation prevents the solidifying shell from sticking to the mould wall — the principal cause of breakouts — by periodically relieving the friction between shell and copper. Mould flux powder is continuously added to the meniscus; it melts on contact with the steel, providing lubrication between the shell and mould wall, insulating the meniscus to prevent freezing, and absorbing floating inclusions. Mould level is controlled automatically to ±1–2 mm using electromagnetic sensors or radioactive source-detector pairs.
Secondary Cooling and Soft Reduction
Below the mould foot rolls, the strand passes through secondary cooling zones where mist sprays (water and air) are applied to the strand surface. Cooling intensity is controlled by zone to maintain a target surface temperature profile that avoids the hot ductility trough (typically 700–900 °C) where the steel is susceptible to transverse cracking during straightening. At the straightening point — where the curved strand is bent back to horizontal — the surface must be above 850–900 °C to avoid cracking. Soft reduction is applied near the final solidification point: small amounts of reduction (typically 2–6 mm) are applied by the last set of driven rolls to close the liquid cone and reduce centreline segregation, which is critical for heavy plates and linepipe steel.
Casting Formats
Continuous casting machines are configured for one of four principal product formats. Slabs — 150–300 mm thick, 800–2,500 mm wide — are produced by curved or vertical-bend machines at integrated flat-rolled mills and account for the largest share of global caster capacity. Blooms (200–400 mm square) and billets (80–180 mm square) are produced on multi-strand long-product casters at mini-mill and merchant bar facilities. Beam blanks — near-net-shape sections for structural I-beams — are cast on dedicated casters and reduce the rolling reduction needed in the structural mill. Thin slab casters (CSP, ISP, QSP) produce 50–90 mm slabs that feed directly into an in-line hot strip mill, characteristic of mini-mill flat-rolled operations.
Breakout: the Critical Quality Failure
A breakout occurs when the solidifying shell ruptures below the mould, releasing liquid steel into the caster — an expensive, time-consuming, and potentially dangerous event. Modern casters use breakout detection systems based on thermocouple arrays embedded in the mould copper, which detect the characteristic temperature signature of a sticking shell before rupture. Automated breakout prediction (BOP) systems can identify a developing sticking event 30–60 seconds in advance and reduce casting speed to allow the shell to thicken. Mean time between breakouts at well-operated slab casters is now measured in months.