2. Scale
• Scale in relation to Hot Mill is an Iron Oxide that
forms at high temperatures.
• The Scale consists of three layers :
FeO – Wustite
Fe3O4 _ Magnetite
Fe2O3 _ Hematite
Wustite forms the inner most layer , Magnetite
the middle one and Hematite the topmost.
3.
4. • The Scale layers can be separated in a process
called De-Lamination.
• De-Lamination occurs either by melting or
changing temperatures in the reheat furnace,
or the mechanical force from the vertical
edgers breaking the outer layer off the slab.
• The scale layers are not chemically bonded.
5. When the De-Lamination occurs between the upper two layers the
descaling layers will remove the top layers and expose any scale
from the lower layers. The scale particles are purple , red or orange
because the outer scale layer protected these inner scale layers
from excessive cooling by the descaling sprays and the two inner
scale layers are more porous with a lower rate of conductivity to
retain the higher temperature.
6.
7. Different Zones in the Furnace
• Recuperating Zone
• Preheating Zone
• Heating Zone 1
• Heating Zone 2
• Soaking Zone
8.
9.
10. The top surface temperature of the slab is higher
than the top furnace temperature when it is
exposed to high flame temperature. The outer
layer of Fe2O3 is caught between the high flame
temperature and two porous insulating layer of
scale which results in higher temperature on the
slab scale surface as compared to the furnace
temperature.
The top surface temperature stays above the
furnace temperature in the heating zone of the
furnace.
11. Primary Red Oxide consists mostly of the
Wustite inner scale layer with a light covering
layer of hematite that has a rusty red colour.
The Red Oxide layer is always a thin and hot
layer because this layer is next to the hot steel
and elongates 100% as much as the steel
underneath.
The Red oxide layer are mostly found on the
edges of the hot rolled coils rather being on
the center.
12.
13. Primary Red Oxide consisting of the inner layer of
primary scale particle is totally protected from
cooling by the descaling sprays and remains almost
at the steel temperature entering the first reduction
pass. The result is the plasticity is about 90% as
high as the slab in the first reduction pass and is
compacted, elongated, and thinner, which allows it
to reach the slab temperature before the next
reduction pass. In the second reduction pass ,the
primary red oxide layer is elongated almost 100%
and the very fractures which may have occurred on
the first reduction pass cannot be seen on coils.
14. High Sulphur content will exhibit much more
rapid scaling. More rapid scaling makes a
thicker scale which is often easier to descale
than thinner layer on low Sulphur steel
grade. The separation point for good
descaling is 0.015 Sulphur or higher.
Silicon retards the scale growth but the steel
grades with more silicon are often the
hardest to descale, and more likely to have
the Red Oxide scale.
15. The Vertical Edgers compresses the edges to create a
“Dogbone” near the edges and the very dense outer layer
of the scale cracks free of the inner scale layer.
This is the reason of the much observed characteristic of
the red scale that it often appears on the sides of the coil
only.
The surface reaction rate is independent of the scale
thickness but is likely to be proportional of the surface
area , it will therefore proceed more rapidly and effectively
the greater the true area of the interaction interface i.e.
the rougher and more irregular the outer surface of the
scale is.
16. The interface bond between the scale
layer and the steel is a function of :
• The original slab or strip surface
texture
•The amount of total oxidation occur
on the steel surface
•The uniformity of the oxidation at
the surface
17. Some Causes of the Red Scale
• High (above 1300 C) heat zone temperature.
• Holding these high temperatures for extended
period of time
• Increasing the temperature in the soaking
zone higher than in the heat zone
• Changing individual zone temperatures to
make the specified transfer bar temperature
• Making frequent zone temperature changes
18. The Preventive Measures
• Reducing the heating zone temperature to a
maximum of 1300 C as measured on the hottest
part of the furnace sidewall in the heating zone.
• Lowering the soak zone set point temperature
below the heating zone temperature.
• Restrict the computer control from using
feedback of the roughing mill temperature to the
average difference of 3-5 slabs to the aim
temperature.
19. • Once the different zone practices are
developed , the changes for different grades
of steel requiring a slab temperature change
needs the heating and soak zones changed
exactly the same number of degrees.
• Eliminate any practice that changes only one
heating zone’s temperature while leaving the
other zone’s the same, when all the slabs
require the same rolling temperature.
• Establish a standard soak zone temperature
for each type of rolling schedule.
20. • Fix every thermocouple tube in a depression
50mm deep x 150mm in diameter with the tip
of the thermocouple tube to be exactly even
with the inside refractory surface of the roof
or sidewalls to make the thermocouple
output readings consistent.
• Each thermocouple furnace has to be in exact
geographical location as the same zone
thermocouples are located in the other
furnaces.
21. Conclusion
Formation of red scale on hot rolled
coils is inevitable. They cannot be
removed completely but can be
reduced to a much lower intensity
level by implementing the preventive
measures given in the report.
22. References
• D.T. Blazevic- Rolled in scale.
• D.T. Blazevic- Scale.
• Y. Ishi, A. Kodoi and I. Wakamastu Mech. Work
steel process.
• Hikaru OKANDA, Tomoki FUKAGAWA,
Haruhiko ISHIHARA, Atsuki OKAMOTO,
Masatoshi AZUMA and Yukio MATSUDA-
Prevention of red scale formation
