Proposal for research on optimization of heat transfer in thin slab continuous casting
processes of steel.
1. Introduction
1.1Technology review
Continuous casting is a process used to solidify molten steel into billets, blooms or slabs
for rolling in the finishing mills. Since its introduction in the 1950s the continuous
casting process has allowed for improvements in yield, quality, productivity and cost in
the steel industry. In this process (Figure1), the molten steel is continuously cast via a
vessel called tundish into a water cooled mold causing a thin shell to solidify next to the
mold walls [1]. The resulting strand is then withdrawn through a set of guiding rolls and
cooled by spraying with a fine water mist. Since the bulk of the metal within the walls of
the strand is still molten, the rollers act as support against the static pressure of the stillsolidifying liquid within the strand. The spraying helps increase the rate of solidification
by extracting heat from the strand surface. The strand becomes completely solidified
when it reaches the metallurgical length. Depending on the design of the casting machine,
a wide range of strands dimensions can be cast (Figure2), ‘slabs’ for flat products such as
plate and strip, ‘blooms’ for sections such as beams, and ‘billets’ for long products such
as wire.
However, the thin shell of the strand can break causing the molten metal in the strand to
spill out and stopping the operation. This is mainly due to a high withdrawal rate or the
metal being too hot. In general the strand will break if final solidification takes place
below the straightening rolls. The breakout has been addressed by vertically oscillating
the mold to separate the solidified steel from the mold walls, in addition additional
separation has been achieved by introducing a powder that acts as a lubricant between the
strand and the mold.
In addition, the solidification of steel has always faced the problem of segregation and
continuous casting has not been immune to it. Dissolved elements generally have a higher
solubility in the liquid phase than in the solid and therefore will tend to come out of the
solution ahead of the solidification front [2]. As the strand solidifies, chunks of solidified
pieces will migrate to the centerline of the strand. This can make the product brittle along
the centerline and result in failure during processing as well as poor product quality.
Thin slab casting in particular has the potential of further improving productivity. Unlike
the conventional casting that produces a slab up to a 10” section; thin slab casters can
produce a slab 2”-3.5” thick. This eliminates the large roughing mills required to work
the thick slabs, and integrates slab production with sheet and strip rolling, greatly
reducing reheating requirements [3]. Recently the use of soft-reduction has allowed a
reduction in the centerline segregation. For soft reduction to have any effect, casting
speed and secondary cooling rate must be chosen so that the metallurgical length is in the
soft reduction zone [4].
1.2 Objective of the study
The main objective of this study is to optimize of the heat transfer in thin slab continuous
casting processes of steel. The proposed study will look at how using soft reduction can
help improve both productivity and material quality at high casting speeds. In order to
reach this goal, we will develop an algorithm that allows us to determine the heat flux
requirements for a desired temperature along the strand thickness.
2. Model formulation
2.1 Heat transfer solidification model
The mathematical model for the heat transfer during the solidification of the strand is
based on the general heat equation for conduction [5]. For a three dimensional heat flux
T
 kT   q (1) where  is the material
in unsteady state it is given as: c p
t
 J 
 kg 
 W 
 ; k is the thermal conductivity 
density  3  ; c p is the specific heat 
 ; T is
m 
mK 
 kg  K 
the temperature (K), t is the time (s) and q represents the term associated to internal heat
generation due to the phase change. However, we will mainly use the two-dimensional
  2T  2T 
T
assumption for this optimization model c
 k  2  2   q (2). Furthermore
t
y 
 x
  2T  2T 
steady state conditions will be assumed reducing (2) to q  k  2  2  (3).
y 
 x
2.2 Optimization model
Alternatives for optimization modeling (gradient methods, particle swarm
methods, genetic algorithms)
3. Milestones and Deadlines
Literature review September 30
Heat transfer model July 30
Optimization model August 30
Testing, verification, validation, production runs October 30
Final report December15
4. References
[1]
[2] A. Scholes, A.W. Smith, S. Riaz, B. Patrick, B. Barber, M.I. McDonald, G.
Stephen… Segregation in continuous casting, Ironmaking & Steelmaking, April 2005,
32, 2, ABI/INFORM Trade & Industry pg. 101
[3]
[4]
5. Figures
Figure1. Continuous Casting Machine
Figure2. Continuous Casting Products