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

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

Flat- and Shape- Rolling Processes Rolling Process

Flat Rolling Process Developed in the late 1500s Developed 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 ingot 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 Keep passing through such rolls until the final thickness is achieved Friction force acts as driving force Friction force acts as driving force The final products include plate, sheet and foil (plate:t>8mm, sheet: t=0.2- 4mm, Foil: 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

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 (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

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

Force and Stress The stress state is similar to that in upsetting 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 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 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 p is a function of h and ϕ For strain hardening material, the flow stress Y f in the expressions corresponds to the strain that the material has undergone at that particular location in the roll gap For strain hardening material, the flow stress Y f 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/h f ratio From the expressions, we can find that the pressure increases with increasing strength of the material, increasing coefficient of friction, and increasing R/h f ratio

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 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 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

Lecture 8 Calculation of the rolling force torque and power  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

Calculation of the Rolling Force Torque and Power Requirements

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) 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? Solution? Force in Hot Rolling Calculation of hot rolling force is important Calculation of hot rolling force is important Two difficulties in calculation Two difficulties in calculation – Estimation of the friction coefficient µ at elevated – Estimation of the friction coefficient µ at elevated temperatures temperatures – Strain rate sensitivity of metals at high temperatures – Strain 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:

Friction It is necessary to have some friction for pulling the trip into the roll gap, but force and power increase with increasing 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 to 0.3; it ranges from 0.2 to 0.7 for hot rolling For cold rolling, µ ranges from to 0.3; it ranges from 0.2 to 0.7 for hot rolling Maximum draft (h 0 -h f ) can be expressed as: Maximum draft (h 0 -h f ) can be expressed as: Δh max = µ 2 R Δh max = µ 2 R α max =tan -1 µ α max =tan -1 µ

Front and back tension Normally, Roll force F can be reduced by various means, such as lower friction, smaller roll radii, small reduction, …. 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 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 σ b ) or at the exit (front tension σ f ). The pressure can be modified as: Tensions in rolling can be applied either at the entry (back tension σ b ) or at the exit (front tension σ f ). The pressure can be modified as: Entry zone Exit zone Entry zone Exit zone

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