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.

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.

Lecture 9 Rolling Practice

Course Hour: 2Basic requirements: Acquainting with rolling practice, spread in flat rolling, the thickness of the sheet is identified by gage number, Hot rolling and cold rollingEmphasis: Flat rolling practiceDifficulties: Spread in flat rolling

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.

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

The thickness of the sheet is identified by gage number

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

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

Hot 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 – ?

Lecture 10 Product quality issue in hot rolling

Course Hour: 2Basic 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 rollingEmphasis: Rolling defects and their causesDifficulties: Rolling defects and residual stress

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 t

ail of slab or plate

Residual stresses

Due to inhomogeneous deformation Small-diameter rolls or small reductions cause surface d

eformation On the other hand, large-diameter rolls or large reduction

s tend to deform the bulk of the material

Leveling

• To improve flatness • The strip is passed through a series of leveling

rolls