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Lecture 7 Rolling history, practice and c ategories and flat Rolling

Course Hour : 2 Basic requirements : Acquainting with rolling history, practice and c ategories ; acquainting with flat rolling process, grasping stress distribution in the flat roll gap Emphasis : Flat rolling process Difficulties : Stress distribution in the flat roll gap.

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Lecture 7 Rolling history, practice and c ategories and flat Rolling

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  1. Course Hour: 2 • Basic requirements: Acquainting with rolling history, practice and categories; acquainting with flat rolling process, grasping stress distribution in the flat roll gap • Emphasis: Flat rolling process • Difficulties: Stress distribution in the flat roll gap Lecture 7 Rolling history, practice and categories and flat Rolling

  2. Rolling Process A process of reducing the thickness or changing the cross-sectional of a workpiece by compressive forces exerted by a pair of rotating rolls

  3. Rolling Process Flat- and Shape-Rolling Processes

  4. Flat Rolling Process • Developed in the late 1500s • 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 ingot • Keep passing through such rolls until the final thickness is achieved • Friction force acts as driving force • The final products include plate, sheet and foil (plate:t>8mm, sheet: t=0.2-4mm, Foil: t<0.2mm), and can be used in a various of fields Flat Rolling Process Rotating rolls reduce the thickness of the incoming ingot

  5. Flat Rolling Process (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on strip surfaces. (c) The roll force, F, and the torque acting on the rolls. The width w of the strip usually increases during rolling

  6. Flat Rolling Process Neutral Point • A point where there are no slip between the workpiece and the roller • The friction of two sides oppose each other at the neutral point • The friction on the entry side must be higher than the exit side • The net friction force and the surface velocity must be in the same direction • Forward slip is defined as: Forward slip

  7. Force and Stress • The stress state is similar to that in upsetting • The calculation in more involved than that of upsetting due to the curved contact surface • The flow stress at the exit is higher than that at the entry Stress on an element in flat rolling. (a) Entry Zone, (b) Exit zone • p is a function of h and ϕ • For strain hardening material, the flow stress Yf in the expressions corresponds to the strain that the material has undergone at that particular location in the roll gap • From the expressions, we can find that the pressure increases with increasing strength of the material, increasing coefficient of friction, and increasing R/hf ratio

  8. Pressure Distribution in the Roll Gap 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 trip in Pressure distribution in the roll gap as a function of coefficient of friction. The area under the curve is the roll separating force per unit width of strip. Note that, as friction increases, the neutral point shits toward the entry. Without friction the rolls slip and the neutral point shifts compeletly toward the exit Pressure distribution in the roll gap as a function of reduction in thickness. Note the increase in the area under the curves with increasing reduction in thickness, thus increasing the roll-separating force

  9. Course Hour: 2 • Basic requirements: Grasping the procedures for the calculations of the rolling force torque and power required for rolling • Emphasis: Calculation of the rolling force torque and power • Difficulties: Calculation of the rolling force torque and power Lecture 8 Calculation of the rolling force torque and power

  10. Calculation of the Rolling Force Torque and Power Requirements

  11. Illustrative Problem A 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, calculate the horsepower required for this operation. (Average flow stress is 125 MPa) Solution? Force in Hot Rolling • Calculation of hot rolling force is important • Two difficulties in calculation – Estimation of the friction coefficient µ at elevated temperatures –Strain rate sensitivity of metals at high temperatures • The average strain rate in flat rolling can be expressed as:

  12. Friction • It is necessary to have some friction for pulling the trip into the roll gap, but force and power increase with increasing friction • For cold rolling, µ ranges from 0.002 to 0.3; it ranges from 0.2 to 0.7 for hot rolling • Maximum draft (h0-hf) can be expressed as: Δhmax= µ2R αmax=tan-1µ

  13. Front and back tension • Normally, Roll force F can be reduced by various means, such as lower friction, smaller roll radii, small reduction, …. • Another effective method is to reduce the apparent compressive yield stress of the material by apply longitudinal tension • Tensions in rolling can be applied either at the entry (back tension σb) or at the exit (front tension σf). The pressure can be modified as: • Entry zone Exit zone

  14. Depending on the relative magnitudes of the tensions applied, the neutral point may shift, and this will affect the pressure distribution, torque and power requirements in rolling • Front tension is controlled by the torque on the coiler (delivery reel). The back tension is controlled by a braking system in the uncoiler (payoff reel) • Tensions are particularly important in rolling thin, high-strength materials

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