Lecture 7 Rolling history, practice and Lecture 7 Rolling history, practice and categories and categories and flat Rollingflat Rolling

Course Hour: 2Basic requirements: Acquainting with rolling history, practice and categories; acquainting with flat rolling process, grasping stress distribution in the flat roll gapEmphasis: Flat rolling processDifficulties: Stress distribution in the flat roll gap

Rolling ProcessRolling Process

A process of reducing the thickness or changing the A process of reducing the thickness or changing the cross-sectional of a workpiece by compressive forcecross-sectional of a workpiece by compressive forces exerted by a pair of rotating rollss exerted by a pair of rotating rolls

Flat- and Flat- and Shape-Shape-Rolling Rolling

ProcessesProcesses

Rolling ProcessRolling Process

Flat Rolling ProcessFlat Rolling Process Developed in the late 1500sDeveloped in the late 1500s Start with slab like ingot (as large as 30 ft by 2 ft by 10 ft)Start with slab like ingot (as large as 30 ft by 2 ft by 10 ft) Pass through two rolls separated by a distance less than the thickness of the ingotPass through two rolls separated by a distance less than the thickness of the ingot Keep passing through such rolls until the final thickness is achievedKeep passing through such rolls until the final thickness is achieved Friction force acts as driving forceFriction force acts as driving force The final products include plate, sheet and foil (plate:t>8mm, sheet: t=0.2-4mm, FThe final products include plate, sheet and foil (plate:t>8mm, sheet: t=0.2-4mm, F

oil: t<0.2mm), and can be used in a various of fieldsoil: t<0.2mm), and can be used in a various of fields

Flat Rolling ProcessFlat Rolling Process

Rotating rolls reduce the thickness of the incoming ingotRotating rolls reduce the thickness of the incoming ingot

Flat Rolling ProcessFlat Rolling Process

(a) Schematic illustration of the flat-rolling process. (b) Friction (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on strip surfaces. (c) The roll force, forces acting on strip surfaces. (c) The roll force, FF, and the torq, and the torque acting on the rolls. The width ue acting on the rolls. The width w w of the strip usually increases of the strip usually increases during rollingduring rolling

Flat Rolling ProcessFlat Rolling ProcessNeutral PointNeutral Point

A point where there are no slip between the workpiece and the A point where there are no slip between the workpiece and the rollerroller

The friction of two sides oppose each other at the neutral pointThe friction of two sides oppose each other at the neutral point The friction on the entry side must be higher than the exit sideThe friction on the entry side must be higher than the exit side The net friction force and the surface velocity must be in the sThe net friction force and the surface velocity must be in the s

ame directioname direction Forward slip is defined as:Forward slip is defined as: Forward slipForward slip

r

rf

V

VV

Force and StressForce and Stress

The stress state is similar to that in The stress state is similar to that in upsettingupsetting

The calculation in more involved The calculation in more involved than that of upsetting due to the than that of upsetting due to the curved contact surfacecurved contact surface

The flow stress at the exit is higher The flow stress at the exit is higher than that at the entrythan that at the entry

Stress on an element in flat rolling. (a) Entry Stress on an element in flat rolling. (a) Entry Zone, (b) Exit zoneZone, (b) Exit zone

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0

' 0 HHeh

hYp H

f

eh

hYp '

.tan2 1

ff h

R

h

RH

p is a function of h and p is a function of h and ϕϕ For strain hardening material, the flow stress YFor strain hardening material, the flow stress Y ff in the expressions corresponds to in the expressions corresponds to

the strain that the material has undergone at that particular location in the roll gathe strain that the material has undergone at that particular location in the roll gapp

From the expressions, we can find that the pressure increases with increasing strFrom the expressions, we can find that the pressure increases with increasing strength of the material, increasing coefficient of friction, and increasing R/hength of the material, increasing coefficient of friction, and increasing R/h ff ratio ratio

Pressure Distribution in the Roll GapPressure Distribution in the Roll Gap

The neutral point shifts toward the exit as friction decreases. If the The neutral point shifts toward the exit as friction decreases. If the friction approaches to zero, the rolls begin to slip instead of pulling the friction approaches to zero, the rolls begin to slip instead of pulling the trip intrip in

Pressure distribution in the roll gap as a function of coPressure distribution in the roll gap as a function of coefficient of friction. The area under the curve is the refficient of friction. The area under the curve is the roll separating force per unit width of strip. Note that, oll separating force per unit width of strip. Note that, as friction increases, the neutral point shits toward thas friction increases, the neutral point shits toward the entry. Without friction the rolls slip and the neutral e entry. Without friction the rolls slip and the neutral point shifts compeletly toward the exitpoint shifts compeletly toward the exit

Pressure distribution in the roll gap as a Pressure distribution in the roll gap as a function of reduction in thickness. Note function of reduction in thickness. Note the increase in the area under the the increase in the area under the curves with increasing reduction in curves with increasing reduction in thickness, thus increasing the roll-thickness, thus increasing the roll-separating forceseparating force

Lecture 8 Lecture 8 Calculation of the rolling force torque and Calculation of the rolling force torque and power power

Course Hour: 2Basic requirements: Grasping the procedures for the calculations of the rolling force torque and power required for rollingEmphasis: Calculation of the rolling force torque and power Difficulties: Calculation of the rolling force torque and power

Calculation of the Rolling ForceCalculation of the Rolling Force

Torque and Power RequirementsTorque and Power Requirements

Illustrative ProblemIllustrative Problem

A 220 mm wide 6061-O Aluminum strip is rolled from a thickness of 25mA 220 mm wide 6061-O Aluminum strip is rolled from a thickness of 25mm to 22mm. If the roll radius is 300 mm. and the roll rpm is 100, calculatm to 22mm. If the roll radius is 300 mm. and the roll rpm is 100, calculate the horsepower required for this operation. (Average flow stress is 125 e the horsepower required for this operation. (Average flow stress is 125 MPa)MPa)

Solution?Solution?

Force in Hot RollingForce in Hot Rolling

Calculation of hot rolling force is importantCalculation of hot rolling force is important Two difficulties in calculationTwo difficulties in calculation – – Estimation of the friction coefficient Estimation of the friction coefficient µ at elevated µ at elevated temperaturestemperatures – –Strain rate sensitivity of metals at high temperaturesStrain rate sensitivity of metals at high temperatures The average strain rate in flat rolling can be expressed as:The average strain rate in flat rolling can be expressed as:

FrictionFriction

It is necessary to have some friction for puIt is necessary to have some friction for pulling the trip into the roll gap, but force anlling the trip into the roll gap, but force and power increase with increasing frictiond power increase with increasing friction

For cold rolling, For cold rolling, µ ranges from 0.002 to 0.3;µ ranges from 0.002 to 0.3; it ranges from 0.2 to 0.7 for hot rolling it ranges from 0.2 to 0.7 for hot rolling

Maximum draft (hMaximum draft (h00-h-hff) can be expressed as:) can be expressed as: ΔΔhhmaxmax= µ= µ22RR ααmaxmax=tan=tan-1-1 µ µ

Front and back tensionFront and back tension Normally, Roll force F can be reduced by various meaNormally, Roll force F can be reduced by various mea

ns, such as lower friction, smaller roll radii, small redns, such as lower friction, smaller roll radii, small reduction, ….uction, ….

Another effective method is to reduce the apparent coAnother effective method is to reduce the apparent compressive yield stress of the material by apply longitumpressive yield stress of the material by apply longitudinal tensiondinal tension

Tensions in rolling can be applied either at the entry Tensions in rolling can be applied either at the entry (back tension (back tension σσbb) or at the exit (front tension ) or at the exit (front tension σσ ff). The p). The pressure can be modified as:ressure can be modified as:

Entry zone Exit zoneEntry zone Exit zone

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0

' 0 HHeh

hYp H

f

eh

hYp '

.tan2 1

ff h

R

h

RH

Depending on the relative magnitudes of the tensions applied, Depending on the relative magnitudes of the tensions applied, the neutral point may shift, and this will affect the pressure dithe neutral point may shift, and this will affect the pressure distribution, torque and power requirements in rollingstribution, torque and power requirements in rolling

Front tension is controlled by the torque on the coiler (delivery Front tension is controlled by the torque on the coiler (delivery reel). The back tension is controlled by a braking system in the reel). The back tension is controlled by a braking system in the uncoiler (payoff reel)uncoiler (payoff reel)

Tensions are particularly important in rolling thin, high-strengTensions are particularly important in rolling thin, high-strength materialsth materials