Rolling Sheet
© Stuller


The production of sheet and strip material uses flat rolling techniques. Material is passed through the gap between two parallel, hardened steel rolls. The rolls rotate and pull the material into the roll gap, squeeze it so that the thickness of the material is reduced, and push the thinner material out the exit side of the roll gap. In general, material passing through a rolling mill does not spread very much in width. It increases greatly in length.

Photomicrographs describe how the grain structure is altered during deformation. Cold rolling will cause grain structures to become stretched and elongated in the direction of rolling. Sometimes this elongated grain is called a rolling texture.

Rolling Tips

1. Material is not flat

a. Check rolls on mill. If the rolls are not exactly the same diameter, the material being rolled will stretch more on one side and cause the stock to "bow" upward or down.

b. Tighten roll gap to apply more force onto the product with the rolls (more reduction). 

c. Material has been cross-rolled in previous operations. Once the material has been rolled in one direction, do not change the rolling direction by rotating the material 90% to the initial rolling direction. If cross rolling must be done, anneal material prior to changing the direction of rolling.

d. Material must be level with the gap between the rolls on both the entrance and exit side of the mill. Feeding or exiting the stock at an angle will create a "bow" in the stock.

e. If the stock is not flat and all conditions during rolling seem to be correct, try passing the stock through the mill several times without changing the gap between the rolls.

2. Edges of stock has cracks

a. Total percentage of reduction between anneals was too great. Total reduction approaching 70% can be risky.

b. Maximum softness was not achieved during annealing. Check time and temperature. 

c. Maximum softness was achieved during annealing, but the alloy has "age-hardening" characteristics. The material increased hardness during cooling because it was not quenched rapidly enough.

d. Flat rolls on mill are worn in the center, and, therefore, edges of stock work harden more rapidly than the center. Measure the thickness on the edges and the center of the sheet. Also a "V" configuration on both ends of the sheet may indicate a problem with rolls.

e. Roll deflection occurs when the rolls bow away from each other while stock is being rolled. This can cause the edges to be reduced further than the center and possibly prematurely crack.

3. Sheet stock is curving

a. Rolls on mill are adjusted tighter on one side. The side that is adjusted tighter will cause the sheet to stretch more on that edge and curve to the opposite side. Make adjustments until sheet straightens.

b. Material is thicker on one edge causing more deformation on thicker side.

c. A large notch or crack on an edge can cause the stock to suddenly curve during the rolling process.

4. Thickness of stock is not uniform down the length

a. Rolls on mill are not round. If true, the distance between the areas of non-uniform thickness will be the same as the circumference of the rolls.

b. Width of the stock varies, which, in turn, varies the degree of resistance against the rolls.

c. Shrinkage voids in cast ingots can cause non-uniform stock thickness.

5. Thickness of stock is not uniform across the width.

a. Roll deflection that causes the edges to reduce further than the center section across the width. Diameter of the rolls and the width need to be matched to the job.

b. Rolls on the mill have become crowned in the middle due to excessive wear on the edges. This will cause the sheet to run from the middle to either side of the roll gap.

6. Poor finish on stock

a. Stock was not properly cleaned prior to rolling. Forces that are used to create plastic deformation are great enough to push dust and liquids into the surface of the stock and leave impressions.

b. Rolls need to be cleaned prior to rolling operations to remove all residues, including oily films that are not visible to the naked eye.

From the Stuller Metals Book

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