Continuous Casting Processes
The main components of a casting machine are shown in Fig.
1. Essentially, a casting machine consists of a liquid metal reservoir and distribution
system (a tundish); a watercooled
mold; secondary cooling zones in association with a containment section;
bending rolls; a straightener; cutting equipment; and
a runout table to cooling beds or directly to a
product transfer area.
A casting machine can have a number of casting strands each
of which is associated with an independent mold, secondary spray water cooling
zone, containment section, etc. The number of strands depends principally on
the shape being cast (slab, bloom, billet etc.) and the heat size.
To start the casting process, a dummy bar (which is
connected to an external mechanical withdrawal system) is inserted in the mold
and positioned so that the top of the dummy bar closes the bottom of the mold
Liquid steel is delivered in a ladle to the casting floor where it is poured at
a controlled rate into the tundish. Liquid metal
flows through nozzles in the bottom of the tundish
and fills the mold. When the liquid metal level in the mold reaches a
predetermined position, withdrawal of the dummy bar is initiated. The
withdrawal speed of the dummy bar is preset based on the casting speed required
or the metal flow rate from the tundish. When the
dummy bar head, which is now attached to the solidified shape being cast,
reaches a certain position in the withdrawal system, it is mechanically
disconnected and the dummy bar removed. The solidified cast shape continues
through the withdrawal system to the cutting equipment.
Fig. 1: Cross section of a slab caster [1]
Solidification of the liquid steel starts in the
water-cooled mold and continues progressively as the strand moves through the
casting machine. Freezing begins at the liquid steel meniscus level in the mold
forming a shell in contact with the walls of the mold. The distance from the
meniscus level to the point of complete solidification within the machine is
called the metallurgical length. Obviously, the point of complete
solidification must occur ahead of the cutting point and in many casters is
ahead of the straightener. Casting conditions are
established such that the strength of the solidified steel shell leaving the
mold is sufficient to withstand the ferrostatic pressure
of the liquid steel in the mold. To prevent sticking of the solidified shell to
the mold wall, the mold is oscillated in a vertical direction. Friction between
the shell and mold is minimized by the introduction of mold lubricants such as
oils or fluxes which form a fluid slag.
Below the mold, additional heat is removed from the strand
in the secondary cooling zones and solidification is completed. The secondary
cooling zones consist of a series of water sprays. Flow rates are closely
controlled to obtain optimum cooling rates and strand surface temperatures.
Support roll units are provided to contain the strand to avoid transverse
movement and to prevent bulging of the hot solidifying shell from internal ferrostatic pressure. The strand cooling and containment
systems are designed, as is the mold, to prevent external and internal defects
in the cast section and insure final product quality.
The secondary cooling and containment area is followed, on
certain types of machine, by a bending unit and a straightener
which is present in all machines.
After straightening, the cast section is cut to the desired
length either by torches or shears. The hot cut lengths are then either
conveyed by a run-out roller table to cooling beds or grouped and transferred
directly to subsequent hot and cold‑rolling operations.
References
[1] The making, Shaping and Treating of Steel, 1985, US
Steel.
[2] The making, Shaping and Treating of Steel, 2002, AISE
Steel Foundation.
