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The Cold
Rolling Mill | Additional
Processes | Temper
Rolling
The Cold Rolling Mill
Cold rolling is accomplished by processing steel
strip through a series of tandem rolling mill stands. Each stand has
vertically stacked rolls that are powered by huge motors to
impart high compressive stresses into the strip. Hot-rolled,
pickled coils are fed into the cold rolling mill from an entry-end
reel and progressively reduced in
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thickness in each stand to achieve the final desired thickness
as the strip exits the last stand. After the last stand, the strip
is recoiled. At this point, the strip is highly cold-worked and not
very useful for most applications. It needs to be annealed to
soften the steel, and make it more formable.
Another method of cold reduction is through the use of a
reversing mill. In this operation, the strip
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is passed back and
forth between mandrels on each side of a single or two-stand mill.
The strip is reduced in thickness on each pass until the final
required thickness is attained. Typical perlent cold reduction
ranges from 60 to 80 percent; for example, reducing a hot rolled
strip of 0.100-inch thick to a 0.030-inch thickness results in 70 percent cold
reduction.
Additional Processing Steps in the
Manufacture of Cold Rolled Sheets
As mentioned previously, the cold-reduced coil, as it exits from
the cold mill, is very hard and possesses very limited
applicability. It cannot be easily bent, it is very stiff and
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exhibits high amounts of springback when bent. Thus, for most
applications, this steel must be annealed (heated to high
temperatures) to soften it, and make it useful.
Annealing is accomplished by two primary methods. One is
batch annealing and the other is continuous
annealing. Batch annealing, as practiced within U.S. Steel,
involves stacking full-size
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coils of cold-reduced material onto 1 to 4 stools that comprise an
annealing base. The stools are then covered with a protective
heat-resistant cover which allows the coils to be maintained within
a protective (non-oxidizing) atmosphere. A large annealing furnace
is then placed over the stools to subject the coils to a closely
prescribed annealing (heating) cycle.
During the heating process, the steel becomes very soft and can
subsequently be used for a multitude of applications. Continuous
annealing involves passing the steel through a high temperature
furnace in the form of a continuous ribbon. That is, the coil is
fed from a payoff reel into the furnace. It reaches a high
temperature during its passage through the furnace. The steel sheet
is then cooled, and recoiled at the exit end of the
furnace.
Whether the steel is batch annealed or continuous annealed, the
specific properties of the steel sheet, after annealing, depends on
the steel chemistry, the temperatures used
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during hot rolling, the amount of cold reduction, and the
annealing cycle (time and temperature).
Whichever method of annealing is used, the steel is maintained
under a protective (non-oxidizing) atmosphere using hydrogen and
nitrogen to prevent oxidizing the steel while it is at high
temperature. In addition to preventing oxidation, the
protective
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atmosphere is designed
to clean the steel by breaking down the oils that are present after
cold rolling and entraining the oil vapors in the hydrogen/nitrogen
gases that are passed through the furnace.
Temper Rolling
Following annealing, the steel coil is most often temper rolled by
passing it through a set of rolls that appear similar to the rolls
used during cold rolling. In fact, temper rolling does impart a
small amount of cold reduction, typically between 0.25 and 1.0
percent.
Various things are accomplished during temper rolling:
- The
tendency for discontinuous yielding, fluting/Lüders lines, is
removed,
- The
surface finish is controlled to provide a range of finishes from
rough matte to a luster finish,
- The
flatness is improved, and The sheet is oiled with a rust
preventative oil.
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