liquid steel is solidified into semi-finished shapes such as slabs, blooms and
billets. Continuous casting now has essentially replaced ingot casting due to
benefits that include increased yield, improved product quality, energy savings
and reduced cost.
Steel Casting After the steel has been refined, it is ready to be cast into ingots
or continuous strips (see the diagram on the following page). In the continuous-casting
process, molten steel is delivered in ladles and poured into a reservoir, or tundish,
from where it is released into the mold by gravity feed. The casting machine can
have either one (single-strand caster) or multiple molds (multi-strand caster).
The steel cools as it passes through the mold and forms a solid shell or "skin."
As the steel proceeds onto the run-out table with a series of hot-handling rollers,
the center of the steel solidifies, yielding a semi-finished shape at a specified
width and thickness. Depending on the type of caster used, billets, blooms, rounds,
thin slabs, or thick slabs are produced. A cutting torch is used at the end of
the roll line to cut the steel to the desired length.
The functions of the caster pouring system are to transfer metal from the ladle
to the caster, control flow to the caster, minimize slag entrainment, minimize
oxygen pickup from the pouring system, cause flotation of inclusions, and minimize
heat loss. Flow is controlled both at the ladle and tundish.
of the harsh operating conditions, the life of many components is less than one
2. Clogging of the caster pouring system is the single largest operational problem
resulting in reduced quality and production delays.
occurs from exposure to air while the molten metal is contained in an uncovered
ladle or tundish or while it is being poured from the ladle to the tundish or
from the tundish to the casting mold. Nitrogen pick-up also occurs and can be
detrimental to the steel chemistry.
There are several
applications for heating technology in continuous casting.
tundish refractory lining (for drying) in order to eliminate hydrogen pickup in
the molten steel and to reduce molten metal heat loss on filling the tundish.
submerged entry nozzle before casting to minimize possibility of metal freezing
at the start of casting.
tundish during casting to minimize temperature variation in molten steel (this
allows casting with as low a superheat as possible).
is complete, Oxy-fuel torches cut the continuous strand to length.
photo on the right shows oxy-gas torches cutting continuously cast slabs to length.
burners (whether air-fuel or Oxy-fuel) are used for both tundish and tundish nozzle
heating. Energy requirements for both needs range from 0.2 to 0.5 Mcf per ton
of cast steel. The tundish nozzle portion of this total is small, about 0.01 Mcf
/ ton and will vary depending on the preheat temperature, the number of ladles
cast in a row and the tundish capacity.
during casting has generally been electric (either resistance, induction or plasma)
for two primary reasons:
1. These systems
can provide rapid heating when liquid metal temperature starts to fall.
2. Pickup of oxygen, hydrogen, or nitrogen from combustion products can be a concern
for some steel grades.
is used for small batches of specialty steels or for end products with certain
shape requirements (e.g., intermediate- and large-bar applications or high performance
bar and tubing applications). Ingot casting also continues to be used by foundries
and specialty steel makers to produce large cross sections or thick plates. During
ingot casting, the molten steel is poured (teemed) into a series of molds and
allowed to solidify to form ingots. After the molds are stripped away, the ingots
are reheated to uniform temperature in any number of reheat furnaces (batch or
continuous) to prepare them for rolling or otherwise forming them into products
because of the reheating step.