1. Roll Deflections
The roll force tend to bend the rolls, leading to a higher thickness in the trip centre
Grinding the rolls so that their diameter at the centre is slightly larger than at edge, called camber
A particular camber is correct only for a certain load and width of strip.
In hot rolling, uneven temperature distribution in the rolls, So camber can also be controlled by varying the location of the coolant (lubricant) on the rolls in hot rolling
Elastic force leads to flattening of the rolls
Reducing the roll force, by such as using an effective lubricant or taking smaller reductions.

2. Lecture 9 Rolling Practice
Course Hour: 2
Basic requirements: Acquainting with rolling practice, spread in flat rolling, the thickness of the sheet is identified by gage number, Hot rolling and cold rolling
Emphasis: Flat rolling practice
Difficulties: Spread in flat rolling

3. Flat Rolling Practice Flat Rolling Practice Cast ingot
Scalp (not always)
Reheat and homogenize
Hot roll in reversing mill
Cut off ends (alligators)
Warm roll
Heat treat to recrystallize (sometimes)
Cool
Cold roll to finish thickness
Heat treat (where appropriate)
Results in material which must be further worked
Flat Rolling Practice
The initial breakdown of an ingot by hot rolling converts the structure to a wrought structure.
The reduction in thickness per pass in practice depends on the type o the workpiece material and its ductility.
Spreading.

4. Spread (increase in width) Flat Rolling
The spread decreases with:
Increasing width-to-thickness ratios of the entering material
Increasing friction
Increasing ratios of roll radius-to trip thickness

5. The thickness of the sheet is identified by gage number

6. Flat Rolling Practice
Converting the coarse-grained, brittle and porous cast structure to a wrought structure by hot rolling
For high-strength alloys reductions are usually small, whereas for soft and nonferrous metal they are high
Temperatures in hot rolling are similar to those in hot forging

7. Independent Variables for Rolling
Roll speed
Draft (amount of thickness reduction) or roll gap
Billet thickness
Billet width
Billet material (not always allowed to select)
Billet temperature
Lubricant
Dependent Variables for Rolling
Roll force
Power
Speed of exiting strip
Final strip temperature
Maximum draft
Roll strip contact length
Sheet or plate shape

8. Hot Rolling Cold Rolling
Advantages
– Stresses lower
– Forces smaller
– Power requirements less
– No work hardening
– Large deformations possible
– Breaks up the cast structure into preferable forms
– Closes porosity
– Sometimes the only way to create sheet
Disadvantages
– Higher friction
– Rolls need to be cooled
– Material handling difficult
– Personnel must be protected from heat
Cold Rolling
Advantages
– Deformations of ~ 50 to 80%
– Work hardening increases strength
– Excellent surface finish
– Excellent tolerance on thickness and shape
Disadvantages
– High forces
– Small reductions give rise to surface stresses and non-uniform stress distributions
– ?

9. Lecture 10 Product quality issue in hot rolling
Course Hour: 2
Basic requirements: Acquainting with different defects in flat rolling; grasping the causes of the defects and the measurements for preventing the occurring of these defects; acquainting with residual stress in flat rolling
Emphasis: Rolling defects and their causes
Difficulties: Rolling defects and residual stress

10. Product Quality Issues
Non-flat material
– Thermal camber
– Roll bending
– Wavy edges (caused by roll bending)
– Zipper and edge cracks (poor ductility)
"Alligatoring" (non-uniform deformation) typically in nose and tail of slab or plate

11. Residual stresses Due to inhomogeneous deformation
Small-diameter rolls or small reductions cause surface deformation
On the other hand, large-diameter rolls or large reductions tend to deform the bulk of the material

12. Leveling To improve flatness
The strip is passed through a series of leveling rolls