CATEGORY:
ROLL SURFACE INDICATIONS
TYPE:
BANDING
POSITION:
WORK ROLL (HOT MILLS)
CHARACTERISTICS
Banding appears on the surface of work rolls in hot strip mill applications as
circumferentially aligned patches that are very coarse (rough) to the eye as well as
touch. The coarse texture within an area of banding comes from a localized variation
in the texture of the roll material with a corresponding variation in oxide thickness.
The coarse surface texture and hence increased friction/bite of the banded areas can
result in localized increased strip surface temperature as well as destabilization of the
strip surface oxide resulting in unfavorable scale formation. Break up of the roll
surface during the banding process can result in small pieces of the roll material being
trapped in the roll bite or being rolled into the surface of the strip. All of these
characteristics can result in poor strip surface quality. This type of issue typically
affects finishing mill work rolls in the early stands.
EXAMPLE
An example of heavy banding (light grey areas) on a pair of HiCr iron work rolls.
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MECHANISM
Banding only occurs in hot strip mill applications and occurs in four distinct stages:
Stage 1 - Firecrazing of the barrel surface: cyclic temperature variations
around the barrel circumference during rolling result in alternating thermal
stress which exceeds the fatigue limit of the material resulting in the formation
of a network of fine cracks (fire crazing) within the strip contact width of the
barrel surface (for a full description, please see Section II.A – Fire crazing).
The severity of the fire crazing is generally gauged by the spacing between the
longitudinal and radial cracks referred to as its cell size. In general the larger
the surface cell size the greater the depth of the fire crazing.
Banding Stage 1
Fire craze cracking
Stage 2 – Break up of the barrel surface: During rolling, contact shear stresses
are generated just below the roll barrel surface that concentrates along the
radial extension of the firecraze cracking (Hertzian Stress – for a full
description, please see Section III.C). When the resultant Hertzian Stress
exceeds the fatigue limit of the shell material along the radial fire craze crack,
secondary contact stress fatigue cracking in initiated and propagated parallel to
the roll barrel surface eventually resulting in a large subsurface network of
radial and tangential cracking. In general, the larger the cell size of the surface
firecrazing from the stage 1 the quicker the break-up of the roll surface is to
develop.
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Stage 3 – Pick-out (comet tails): Once the subsurface cracking network
propagates to a point where linking of radial and tangential cracking occurs,
individual cells of shell metal can then begin to separate away from the parent
roll material (pick-out). As individual cells of shell metal separate from the
roll, the oxide layer remains attached to the rolling surface of the cell and
forces a small amount of the surrounding oxide to peel away with the cell in
the opposite direction of roll rotation. As a result the indication has the
appearance of a shallow pit within the roll material with a short “comet tail” of
peeled oxide trailing behind it. Re-oxidation of the roll surface within the pit
as well as the “comet tail” area will occur as rolling continues. The oxide layer
will not however be sufficient to completely fill in the “pit” resulting in a
depression or rough area of the roll surface.
Banding Stage 3
Individual fire craze cells break out from the parent material forming a “comet mark”
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Multiple comet marks forming at the intersection of stall band firecracks.
Stage 4 – Banding: With continued rolling the break-up of the barrel surface
becomes more severe with multiple individual cells becoming separated from
the barrel surface with corresponding removal/peeling of the oxide layer. As
larger strips of oxide are peeled away they take with it more of the firecraze
cells accelerating the roughening of the barrel surface. Because the whole
process of pick-out of the roll material with resultant peeling of the oxide layer
occurs in a continuous but nonuniform manner, the resultant roll surface
becomes very rough. As with Stage 3 re-oxidation of the banded layers occurs
but will not be sufficient to fill in or return the area to its original smooth
surface condition. It is typically seen that the appearance and texture of the roll
surface at differing locations throughout the strip contact width can be in
different stages either roll material pick-out, peeling and rebuilding of the
oxide layer.
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Banding Stage 4
Multiple fire craze cells break out and initiate the formation of a band
Multiple areas of banded roll oxide visible within the rolling contact width. Adjacent
to the main banded areas the earlier stages of the process such as “comet marks” can
also be seen forming.
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Close up within a banded area of the roll barrel. The rough surface present is
generated as a result of fire craze cells breaking out from the parent material followed
by re-oxidation of the surface. Once banding occurs a good smooth roll surface cannot
be re-established without regrinding.
PREVENTION
Banding can be retarded or prevented by the following:
 Minimize the severity of the thermal cycle generated on the roll barrel
surface during operation (i.e. reduce the maximum to minimum
temperature difference and rate of change that the roll surface is exposed
to during one complete rotation). This can generally be controlled through
the cooling that the roll is exposed to during operation. In general, the
greater the thermal stress differential the roll surface is exposed to during
each rotation (i.e. the more severe the roll or strip cooling), the fewer total
number of cycles will be required for thermal fatigue cracks to develop
and propagate.
The following operational factors can all influence the thermal stress
differential at the roll surface:





Interstand cooling and or Skin cooling
Roll Lubrication
Reduction
Roll Coolant volume and distribution
Rolling Pace
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 Reduce the total amount of time the roll is in the mill (campaign length).
This will minimize the total number of thermal and mechanically induced
stress cycles on the roll surface.
 Increase the hot strength of the roll material. This can be accomplished by
either increasing the manufactured hardness of the roll or by switching to
a roll material that naturally exhibits a higher hot strength (such as HSS).
By increasing in the hot strength, the roll will require a greater number of
stress cycles the roll is exposed to before thermal fatigue will initiate. This
means that the rolls will be able to be run longer before banding will
occur.
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