Continuous Casting

What is Continuous Casting?

Continuous casting (also called strand casting) is the process by which molten steel is solidified into a semi-finished billet, bloom, or slab for subsequent rolling in finishing mills. It replaced the older ingot casting process and now accounts for over 96% of global steel production.

The key advantage of continuous casting is that liquid steel is solidified and formed into a continuously moving strand rather than being poured into discrete ingot molds—dramatically improving productivity, yield, and product uniformity.

Machine Types

Continuous casting machines are classified by the shape of the product they produce:

• Slab casters produce wide, flat slabs (typically 150–250 mm thick, 800–2200 mm wide) used for flat-rolled products such as sheet, plate, and strip.
• Bloom casters produce square or rectangular sections (typically 150×150 mm to 400×600 mm) used for structural shapes, rails, and seamless pipe.
• Billet casters produce small square sections (typically 100×100 mm to 160×160 mm) used for bars, wire rod, and small structural shapes.
• Thin slab casters produce near-net-shape thin slabs (40–100 mm thick) that can be rolled inline, reducing energy consumption and capital cost.

Process Description

In a conventional continuous casting machine:

1. Liquid steel is tapped from the steelmaking furnace into a ladle, where it may receive additional refining treatments.
2. The ladle is placed above a tundish, a shallow refractory vessel that serves as a buffer and distributes steel to one or more casting strands.
3. Steel flows from the tundish through a submerged entry nozzle (SEN) into a water-cooled copper mold, where solidification begins and a thin steel shell forms against the mold walls.
4. The partially solidified strand is withdrawn from the bottom of the mold at a controlled casting speed and supported by rolls to prevent bulging.
5. Secondary cooling water sprays and radiation cooling further solidify the strand as it travels through the machine.
6. The fully solidified strand is straightened (in curved machines), cut to length by torch cutters or shears, and transferred to rolling mills or storage.

Key Process Parameters

Successful continuous casting requires precise control of numerous parameters:

• Casting speed: 0.8–2.5 m/min for slabs, up to 6 m/min for billets
• Steel superheat: typically 15–35°C above liquidus temperature
• Mold cooling water flow rate and temperature rise
• Mold oscillation: frequency and stroke to prevent sticking
• Mold flux: type and consumption rate
• Secondary cooling: water flow rates in each cooling zone
• Electromagnetic stirring (EMS): frequency and current

Defects and Quality Issues

Continuous casting can produce a range of surface and internal defects that affect downstream processability and final product quality. Common defects include:

• Longitudinal and transverse surface cracks
• Oscillation marks and associated hook cracks
• Subsurface pinholes from entrapped argon bubbles
• Inclusion entrapment near the surface
• Centerline segregation and porosity
• Internal cracks from thermal stress and mechanical bending
• Breakouts caused by thin shell failure in the mold

CCC research aims to understand the root causes of these defects and develop models and process recommendations to prevent them.