The roller hearth tunnel
furnace is the critical link between the casting unit(s) and
the rolling mill. This furnace has two primary
functions. The first is to heat the incoming slab to
rolling temperature and then equalize temperatures. The
second function is to act as a buffer zone between the
casting operation and the rolling operation.
The furnace length is
determined by several factors — mill productivity, slab
heating time, maximum slab dimensions and buffer time
requirements. Usually these furnaces are designed to hold
three to four full length slabs.
Typically, a slab cut to
length enters the furnace at casting speed and is heated for
the appropriate time. It then is transferred at higher speed
to the discharge zone where its temperature becomes very
uniform. Finally, the slab is discharged at rolling speed
into the first stand of the Hot Strip Mill.
If there is a delay at the
Hot Strip Mill, a roll change for example, the slab is held
in the discharge zone and oscillated until the mill is again
ready to roll.
These furnaces have a high
degree of computer control. The control must receive
information from the casting operation and the mill
operation and then adjust the furnace operation to provide a
seamless match. This sophisticated control includes product
tracking, level 2 thermal modeling and process simulation.