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The production of
sheet and strip material uses flat rolling techniques. Material is
passed through the gap between two parallel, hardened steel rolls.
The rolls rotate and pull the material into the roll gap, squeeze it
so that the thickness of the material is reduced, and push the
thinner material out the exit side of the roll gap. In general,
material passing through a rolling mill does not spread very much in
width. It increases greatly in length.
Photomicrographs
describe how the grain structure is altered during deformation. Cold
rolling will cause grain structures to become stretched and elongated
in the direction of rolling. Sometimes this elongated grain is called
a rolling texture.
Rolling Tips
1. Material
is not flat
a. Check rolls on
mill. If the rolls are not exactly the same diameter, the material
being rolled will stretch more on one side and cause the stock to
"bow" upward or down.
b. Tighten roll
gap to apply more force onto the product with the rolls (more reduction).
c. Material has
been cross-rolled in previous operations. Once the material has been
rolled in one direction, do not change the rolling direction by
rotating the material 90% to the initial rolling direction. If cross
rolling must be done, anneal material prior to changing the direction
of rolling.
d. Material must
be level with the gap between the rolls on both the entrance and exit
side of the mill. Feeding or exiting the stock at an angle will
create a "bow" in the stock.
e. If the stock is
not flat and all conditions during rolling seem to be correct, try
passing the stock through the mill several times without changing the
gap between the rolls.
2. Edges of
stock has cracks
a. Total
percentage of reduction between anneals was too great. Total
reduction approaching 70% can be risky.
b. Maximum
softness was not achieved during annealing. Check time and temperature.
c. Maximum
softness was achieved during annealing, but the alloy has
"age-hardening" characteristics. The material increased
hardness during cooling because it was not quenched rapidly enough.
d. Flat rolls on
mill are worn in the center, and, therefore, edges of stock work
harden more rapidly than the center. Measure the thickness on the
edges and the center of the sheet. Also a "V" configuration
on both ends of the sheet may indicate a problem with rolls.
e. Roll deflection
occurs when the rolls bow away from each other while stock is being
rolled. This can cause the edges to be reduced further than the
center and possibly prematurely crack.
3. Sheet
stock is curving
a. Rolls on mill
are adjusted tighter on one side. The side that is adjusted tighter
will cause the sheet to stretch more on that edge and curve to the
opposite side. Make adjustments until sheet straightens.
b. Material is
thicker on one edge causing more deformation on thicker side.
c. A large notch
or crack on an edge can cause the stock to suddenly curve during the
rolling process.
4.
Thickness of stock is not uniform down the length
a. Rolls on mill
are not round. If true, the distance between the areas of non-uniform
thickness will be the same as the circumference of the rolls.
b. Width of the
stock varies, which, in turn, varies the degree of resistance against
the rolls.
c. Shrinkage voids
in cast ingots can cause non-uniform stock thickness.
5.
Thickness of stock is not uniform across the width.
a. Roll deflection
that causes the edges to reduce further than the center section
across the width. Diameter of the rolls and the width need to be
matched to the job.
b. Rolls on the
mill have become crowned in the middle due to excessive wear on the
edges. This will cause the sheet to run from the middle to either
side of the roll gap.
6. Poor
finish on stock
a. Stock was not
properly cleaned prior to rolling. Forces that are used to create
plastic deformation are great enough to push dust and liquids into
the surface of the stock and leave impressions.
b. Rolls need to
be cleaned prior to rolling operations to remove all residues,
including oily films that are not visible to the naked eye.
From the Stuller
Metals Book
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