/ Steel (Ferrous): Reheat Furnaces
heat semi-finished shape prior to rolling or forming into the final product (plate,
strip, structural shape, etc.). Many reheat furnace designs exist. However, the
most commonly utilized furnace types are:
- Steel is pushed in the charge end up against the steel already in the furnace.
As one is pushed in (cold or at lower temperature) another is pushed out (ready
for rolling or forming)
Beam - Steel is actually raised up, moved forward, and lowered back down
in the furnace. In this manner, material is "walked" through the furnace.
- Circular in design. Steel is placed into the furnace on a hearth that rotates
and the material is heated as it travels on the hearth.
at mini-mills generally utilize natural gas fired burners (although some limited
induction systems have been utilized). Reheat furnaces in integrated steel mills
also use natural gas fired combustion systems. However, where coke oven gas is
available, it is common to use coke oven gas in one or more heating zones or to
mix natural gas and coke gas in all firing zones.
are the most common type of reheat furnace used by steel companies. However, for
the more recent installations, walking beam furnaces are more commonly specified.
also occur during the rolling or shaping process. Special edge heating stands
for strip, slabs or long bar products have been installed. In this area oxy-fuel
natural gas systems have been demonstrated and can effectively compete with induction.
of steel is generally accomplished at temperatures in the range of 2,000 to 2,200°F.
Traditionally, a significant volume of natural gas has been consumed in reheat
operations. Many older pusher and walking beam furnaces have been upgraded to
meet higher productivity needs, to lower energy use and to reduce emissions.
pusher furnaces are generally both top and bottom fired (burners located above
and below the hearth line) and equipped with a water-cooled and insulated skid
support system. The chief disadvantage of a pusher design is that the skid system
(that the steel being heated slides on) will cause cold spots (skid marks) to
form on the bottom of the steel being heated. These marks, if not minimized, are
detrimental to the subsequent rolling or shaping operation. Walking beam furnaces
also utilize top and bottom firing. However, they stagger the location of the
support beams and, therefore, reduce or eliminate colder areas. Also no skid marks
are created. Additionally, unlike pusher furnaces, which allow cool spots to form
where they push against each other; walking beam furnaces keep billets separated.
gas consumption will vary based on the material size (thin slab versus thick),
if the material is charged into the reheat furnace cold or if a hot charging practice
with shapes direct from the continuous caster is used, and how well the equipment
is operated and maintained.
A photograph of a thin slab emerging from a caster is shown to the right. Current
practice is to locate an equalizing (or tunnel furnace) close to the exit point
of the caster.