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metal-rolling process and equipment, - - Coggle Diagram
metal-rolling process and equipment
introduction
rolling: is the process of reduction the cross section of a long workpiece by compressive forces applied through a set of rolls
the flat-rolling process
reducing roll force
roll forces can cause deflection and flattening of the rolls
the columns of the roll stand may deflect under high roll forces
roll forces can be reduced by
reducing friction at the roll-workpiece interface
using smaller diameter rolls
reduce the contact area
rolling at elevated temperature
applying front and/or back tensions to the strip
geometric considerations
roll forces will bend the rolls elastically during rolling
when the roll bends, the strip has a constant thickness along it`s width
the heat generated by plastic deformation cause rolls to be slightly barrel shaped (thermal camber)
rolls forces also tend to flatten the rolls elastically
spreading
increase in width is called spreading
spreading increase with
decreasing width to thickness ratio of the entering strip
increase friction
decreasing ratio of the roll radius to the strip thickness
vibration and chatter
vibration and chatter have effects on product quality and the productivity of metalworking operations
chatter defined as self-excited vibration
occur in rolling, extrusion, drawing, machining, grinding operations
chatter result from interaction between the structural dynamics of the mill stand and the dynamics of the rolling operations
chatter can be reduced by increasing the roll radius, strip-roll friction and incorporating dampers in the roll supports
flat-rolling practice
initial rolling steps (breaking down) of the material is done by hot rolling
a cast structure is dendritic and brittle and porous
hot rolling converts the cast structure to a wrought structure with fine grains and enhanced ductility
product of the first hot-rolling operation is called a bloom , a slap, a billet
to improve flatness, the rolled strip goes through a series of leveling rolls
defects in rolled plates and sheets
surface defects may be caused by inclusions and impurities in the original cast material
wavy edges on sheets are the result of roll bending
they are undesirable as they compromise surface appearance and adversely affect strength and formability and other manufacturing characteristics
cracks are due to poor material ductility at rolling temperature
defects may be present on the surfaces or there may be internal structural defects
other characteristics of rolled metals
residual stresses
develop in rolled plates and sheets due to nonuniform deformation of materials in roll gap
dimensional tolerances
thickness tolerance for cold-rolled sheets range from +or - 0.1 to 0.35
flatness tolerance are within +or- 15mm/m for cold rolling and +or- 55mm/m for hot rolling
surface roughness
cold rolling can produce a very fine surface finish
cold-rolled sheets products may not require additional finish operations
rolling mills
automated mills produce close-tolerance, low cost and high quality plates and sheets at high production rates
two-high rolling are used of hot rolling in initial breakdown passes (cogging mills) on cast ingots and continuous casting
in tandem rolling the strip is rolled continuously through a number of stands to thinner gages with each pass
roll materials
basic requirements for roll materials are strength and resistance to wear
forged-steel rolls have higher strength, stiffness and toughness than cast-iron rolls
rolls made for cold rolling should not be used for hot rolling as they may crack thermal cycling and spelling
lubricants
hot rolling of ferrous alloys do not need lubricants
water-based solutions are used to cool the rolls
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