Rolling

Rolling
Objective
To perform rolling process on an lead bar in order to observe the change in both the cross-sectional area and
the general shape.
Theory
1. Definition
Flat rolling or Rolling is defined as the reduction of the cross-sectional area of the metal stock, or the general
shaping of the metal products, through the use of the rotating rolls [1]. It allos a high degree of closed-loop
automation and ver! high speeds, and is thus capable of providing high-"ualit!, close tolerance starting
material for various secondar! sheet metal orking processes at a lo cost [1].
2. Schematic Drawing of Rolling Process
Figure 1. Rolling #rocess [$]
The rolls rotate as illustrated in Figure 1. to pull and simultaneousl! s"uee%e the ork beteen them. The
basic process shon in Figure 1 is flat rolling, used to reduce the thickness of a rectangular cross section.
Figure $. &arious configurations of rolling mills' (a) to high, (b) three high, (c) four high, (d) cluster mill,
and (e) tandem rolling mill [$].
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&arious rolling mill configurations are available to deal ith the variet! of applications and technical
problems in the rolling process. The basic rolling mill consists of to opposite rotating rolls and is referred
to as a to-high rolling mill (Figure $a). In the three-high configuration Figure $(b), there are three rolls in a
vertical column, and the direction of rotation of each roll remains unchanged. To achieve a series of
reductions, the ork can be a passed through from either side b! raising or loering the strip after each pass.
The e"uipment in a three-high rolling mill becomes more complicated, because an elevator mechanism is
needed to raise and loer the ork [$].
Roll-ork contact length is reduced ith a loer roll radius, and this lads to loer forces, tor"ue, and poer.
The four-high rolling mill uses to smaller diameter rolls to contact the ork and to backing rolls behind
them. *nother roll configuration that allos smaller orking rolls against the ork is the cluster rolling mill.
To achieve higher throughput rates in standard products, a tandem rolling mill is often used. This
configuration consists of a series of rolling stands. +ith each rolling step, ork velocit! increases, and the
problem of s!nchroni%ing the roll speeds at each stand is significant [$].
3. General Overview of Process
The primar! ob,ectives of the flat rolling process are to reduce the cross-section of the incoming material
hile improving its properties and to obtain the desired section at the e-it from the rolls. The process can be
carried out hot, arm, or cold, depending on the application and the material involved. The rolled products
are flat plates and sheets. Rolling of blooms, slabs, billets, and plates is usuall! done at temperatures above
the recr!stalli%ation temperature (hot rolling). .heet and strip often are rolled cold in order to maintain close
thickness tolerances.
/asicall! flat rolling consists of passing metal beteen to rolls that revolve in opposite directions, the
space beteen the rolls being somehat less than the thickness of the entering metal. /ecause the rolls
rotate ith a surface velocit! e-ceeding the speed of the incoming metal, friction along the contact interface
acts to propel the metal forard. The metal is s"uee%ed and elongated and usuall! changed in cross section.
The amount of deformation that can be achieved in a single pass beteen a given pair of rolls depend on the
friction conditions along the interface. If too much is demanded, the rolls ill simpl! skid over stationer!
metal. Too little deformation per pass results in e-cessive cost.
Rolling involves high comple-it! of metal flo during the process. From this point of vie, rolling can be
divided into the folloing categories [0]'
• 1niform reduction in thickness ith no change in idth' 2ere, the deformation is in plane strain,
that is, in the directions of rolling and sheet thickness. This t!pe occurs in rolling of strip, sheet, or
foil.
• 1niform reduction in thickness ith an increase in idth' 2ere, the material is elongated in the
rolling direction, is spread in the idth direction, and is compressed uniforml! in the thickness
direction. This t!pe occurs in the rolling of blooms, slabs, and thick plates.
• 3oderatel! non-uniform reduction in cross section' 2ere, the metal is elongated in the rolling
direction, is spread in the idth direction, and is reduced non-uniforml! in the thickness direction.
• 2ighl! non-uniform reduction in cross section' 2ere, the reduction in the thickness direction is
highl! non-uniform. * portion of the rolled section is reduced in thickness hile other portions ma!
be e-truded or increased in thickness. *s a result, in the idth direction metal flo ma! be toard
the center [0].
Hot Rolling
$
The distinctive mark of hot rolling is not a cr!stalli%ed structure, but the simultaneous occurrence of
dislocation propagation and softening processes, ith or ithout recr!stalli%ation during rolling. The
dominant mechanism depends on temperature and grain si%e. In general, the recr!stalli%ed structure becomes
finer ith loer deformation temperature and faster cooling rates and material of superior properties are
obtained b! controlling the finishing temperature [1].
2ot rolling offers several advantages [1]'
1) Flo stresses are lo, hence forces and poer re"uirements are relativel! lo, and even ver! large
orkpieces can be deformed ith e"uipment of reasonable si%e.
$) 4uctilit! is high5 hence large deformations can be taken (in e-cess of 667 reduction).
0) 8omple- part shapes can be generated.
The upper limit for hot rolling is determined b! the temperature at hich either melting or e-cessive
o-idation occurs. 9enerall!, the ma-imum orking temperature is limited to :;<8 belo the melting
temperature. This is to allo the possibilit! of segregated regions of loer melting material [=].
Cold Rolling
8old rolling, in the ever!da! sense, means rolling at room temperature, although the ork of deformation
can raise temperatures to 1;;-$;;<8. 8old rolling usuall! follos hot rolling. * material sub,ected to cold
rolling strain hardness considerabl!. 4islocation densit! increases, and hen a tension test is performed on
this strain-hardened material, a higher stress ill be needed to initiate and maintain plastic deformation5
thus, the !ield stress increases. 2oever, the ductilit! of the material > as e-pressed b! total elongation and
reduction of area > drops because of the higher initial dislocation densit!. .imilarl!, strength coefficient rises
and strain-hardening e-ponent drops. 8r!stals (grains) become elongated in the direction of ma,or
deformation [1].
8old rolling has several advantages [1]'
1) In the absence of cooling and o-idation, tighter tolerances and better surface finish can be obtained.
$) Thinner alls are possible.
0) The final properties of the orkpiece can be closel! controlled and, if desired, the high strength
obtained during cold rolling can be retained or, if high ductilit! is needed, grain si%e can be
controlled before annealing.
=) ?ubrication is, in general, easier.
Rolling Problems and Defects
The main problem during rolling process is the calibration of rollers. This calibration faults ma! occur in
case of used bearings and ma! affect the thickness of parts. * simple classification is as here belo'
a. ?engthise @ccurring 4efects
8hange of rollers speed
3aterial temperature
Roller temperature
Inlet thickness
3aterial properties
Accentric and conical rollers
1sed bearings
Transversall! @ccurring 4efects
#arallel position of rollers
.urface geometr! of rollers
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