Rolling is an indirect compression process. Normally the only force or stress applied is
the radial pressure from the rolls. This deforms the metal and pulls it through the roll gap.
The process can be compared to compression or forging but differs in two respects in that
compression takes place between a pair of platens at various inclinations to each other,
and that the process is continuous, Fig. 1.
Fig.1.
Rolling is the most widely used deformation process and for the reason that there are so
many versions the process has its own classification. This can be according to the arrangement
of the rolls in the mill stand or according to the arrangement of the stands in sequence.
Rolling mills are classified as in Fig. 2.
Fig.2.
The two-high mill was the first and simplest but production rates tended to be low because
of the time lost in returning the metal to the front of the mill. This obviously led to
the reversing two-high mill where the metal could be rolled in both directions. Such a
mill is limited in the length that it can handle, and if the rolling speed is increased,
the output is almost unchanged because of the increased time spent in reversing
the rotation at each pass. This sets an economic maximum of about 10 meters.
The next obvious development was the three-high mill, which has the advantages of both
the two high reversing and non-reversing mills. Such a mill must, of course, have
elevating tables on both sides of the rolls. The roll gap on a three-high mill cannot
be adjusted between passes, therefore grooves or passes must be cut into the roll face
to achieve different pass reductions.
All three kinds of mill suffer from the disadvantage that all stages of rolling are
carried out on the same rolled surface and the surface quality of the product tends
to be low. Roll changes on such mills are relatively frequent and time consuming.
This type of mill is therefore used for primary rolling where rapid change of shape is
required, even at the expense of surface quality.
Four-high mills are a special case of two-high, and in an attempt to lower the rolling
load, the work roll diameter is decreased.
There is, however, a risk of roll bending which is avoided by supporting the small
work rolls by larger backing rolls. The backing roll diameter cannot be greater than
about 2-3 times that of the work rolls, and as the work roll diameter is decreased more
and more (to accommodate processes with exceedingly high rolling loads) the size of the
backing rolls must also decrease. A point is reached when the backing rolls themselves
begin to bend and must be supported hence the ultimate design - the cluster mill.
The principal criticism of the traditional mill is this tendency for roll bending due
to its inherent design - the beam principle. Sendzimir proposed a design which
eliminated this limitation based on the castor principle where the work roll is
supported over ali its face by an array of backing rolls. This principle can be applied
to much mills and an installation for rolling stainless steel 1600 mm wide is fitted
with work rolls 85 mm diameter.
Continuous rolling mills can be classified according to the arrangement of stands or passes.
These are in line in a continuous mill and line abreast in a looping or cross-country mill.
Looping and cross-country mills require the workpiece to be bent or turned between stands
and are used therefore for rolling rods, rails or sections. Continuous mills are used for
plates, strip or sheets. They all require a large capital outlay and are only justified when
a large demand for the product is guaranteed.
It is possible to derive an expression for this friction force. Pressure acts radially on
the ends of this element, and if the element is located between the point of entry and the
neutral point a frictional force acts toward the neutral point. The radial pressure has a
horizontal component which tends to reject the metal and prevent it from entering the rolls,
whilst the friction force has a horizontal component dragging the metal inward. Whether the
metal passes through the rolls depends upon the values of the two horizontal force components.
Primary rolling is a process where large maximum reductions are required in order that the metal
can be deformed quickly and cheaply. Such mills have large diameter rolls with surfaces that are
roughened or ragged to increase the coefficient of friction.
The rolling load can be minimized by making the radius as small as possible and the roll surface
as smooth as possible. This principle is used in the design of cluster mills which are used
extensively for foil rolling and consist of small work rolls supported by larger back-up rolls to
prevent bending. Even with such mills the rolling loads can still be excessive and recourse is made
to devices which apply front and back tension to the metal being rolled.
Foil rolling and finishing mills are generally very different from primary mills which as already
seen tend to use large diameter rolls with roughened surfaces.
It is an essential of metal-deformation processes that the tool is only loaded elastically,
while the workpiece is flowing plastically. This elastic deformation is generally so small that it
can be ignored, but this is not the case in rolling. There are two reasons. One is that rolling
loads and stresses can be very large, especially when the workpiece is thin and work-hardened.
The other is that the tool in rolling comprises the whole mill-rolls and housing with overall
dimensions measurable in meters. This combination can result in very large strains due to elastic
deformation divided between mill stand extension "mill spring", roll flattening and roll bending.
Roll flattening. The workpiece passing between a pair of rolls is compressed by the radial
stress in them, but the reaction is transferred to the mill bearings and housing, which are capable
of only limited yield because of their large dimensions. If an attempt is made to compress thin
hard material further, the reaction becomes so large that the rolls deform elastically and the
radius of curvature of the arc of contact is increased. The extent of this flattening depends
on the magnitude of the reaction stress and the elastic constants of the rolls.
Roll flattening has another effect in that for a given mill there is a minimum gauge below which it
is not possible to roll. Any attempt to do so results in greater deformation of the rolls, without
any plastic deformation of the strip. With thin gauges as already seen the friction hill becomes
very large producing reaction stresses in the arc of contact which exceed the yield stress of
the rolls, therefore it is easier to deform the rolls than the metal. As long as the mill is running
the rolls will remain circular, but if the load is not removed when it is stopped, deformation will
take place to flatten the surface over the area of contact between the rolls.
Attempts to avoid or limit roll bending have involved ways of decreasing the rolling load. This has
resulted in small work rolls and four-high mills. But even with these mills a certain amount of roll
bending still occurs and is accommodated by cambering the rolls, i.e. making them barrel shaped.
With multistand continuous rolling, interstand tension is adjusted to maintain the rolling load to
a constant value and so achieve a flat surface. This is an important aspect of shape control in
the rolling of strip.
A recent development has been the introduction of hydraulic jacks onto the roll necks thereby
altering the roll camber by actually bending the rolls. Results to date indicate that this method
will be very successful in controlling strip shape.
All the methods described so far have involved continuous rolling where front and back tension
or interstand tension can be used. With single sheet rolling this technique for controlling
rolling load cannot be used and therefore the problem of shape control is tackled in another way.
Mill spring or plastic distortion. The reaction to rolling load is called the roll
separating force and if the rolls were not held in the mill housing they would indeed separate
and reduction of metal would not be possible. The upper roll pushes the top of the housing
upwards whilst the bottom roll pushes the base of the housing downwards. The housing is
therefore subjected to a tensile stress, which is obviously below the yield stress of the
cast steel normally used, but there is a measurable elastic deformation.
The extent depends upon (a) the rolling load, (b) the cross-sectional area of the housing,
and (c) the height of the housing. If the extent of this deformation is small the mill is
said to be hard or rigid, whilst if it is large, the mill is said to be soft or springy.
It is a characteristic of the mill and can be determined in the following way. The mill
is set to a constant roll gap and a series of different pieces of metal are rolled.
These produce different rolling loads which are measured. The rolling loads can be varied
either by using different gauges of the same metal or by using different metals.
A graph is drawn relating rolling load to gauge, the gauge being found by measuring
the thickness of the rolled pieces.