Plastic- Case Study 4: Gauge Control During Cold Rolling

Question 1

The two groups of deformation processes

Answer 1

Primary working operations: rolling, extrusion, forging.
Secondary working operations: wire drawing, machining.

Question 2

What is rolling?

Answer 2

The process of reducing the thickness (gauge) or cross section of a workpiece by compressive forces applied through a set of rolls.

Question 3

Types of rolling

Answer 3

Basic operation is flat rolling where the finished product is plate or sheet.
There is also section rolling for non-flat products like rail tracks.

Question 4

Four considerations when rolling

Answer 4

Is the workpiece too thick to go through roll gap?
Does the mill have enough power/torque for the operation?
Separating force will separate rolls (mill spring) so distance between rolls needs to be less than the desired thickness of the product.
Is the product flat?

Question 5

What are two high and four high mills?

Answer 5

Two high is one roll on either side of roll gap.
Four high is two rolls (on top of each other) on either side of roll gap.
These are used for the initial breakdown of cast ingots

Question 6

Smaller diameter rolls

Answer 6

More preferable because the smaller the radius the lower the roll force. But smaller diameter rolls require back up rolls to increase stability

Question 7

What types of mills are suitable for cold rolling of high strength materials?

Answer 7

Cluster or sendzimir mills as they provide rigidity

Question 8

Describe the work roll

Answer 8

Can be very small in diameter and usually made of or coated with tungsten carbide or a similar material

Question 9

Describe the layout of a cluster mill

Answer 9

Aka Sendzimir or Z mill. Strip of metal goes through the gap between the two work rolls. For each work roll there are two first intermediate rolls, then three second intermediate rolls (these are driven rolls), then four bearing shafts with backing bearings on their circumference. All inside a housing. Very rigid and used in rolling thin sheet of high strength materials such as stainless steel

Question 10

Describe the Steckel mill

Answer 10

Uses idling rolls in which the torque on the roll is effectively 0. The metal is cooled and uncoiled either side of the work rolls for multiple passes through the work rolls. Can control the tension. Used for making thin plate or thick sheet

Question 11

What are the requirements for roll materials?

Answer 11

Strength and wear resistance. Means common materials are steel and cast iron

Question 12

How are rolls modified for hot rolling?

Answer 12

The rolls are rough and have notches or grooves in order to pull the metal through

Question 13

What is done to rolls for coils rolling?

Answer 13

They are ground to a very fine finish and may even be polished to improve surface quality

Question 14

Plates thickness and uses

Answer 14

Thickness greater than 6mm. Used for boilers, nuclear vessels, bodywork for ships and large construction vehicles, large construction girders for bridges and other large structures.

Question 15

Sheet thickness and uses

Answer 15

Rolled material with thickness less than 6mm. Used for car bodies, packaging, aircraft fuselages. Produced in flat pieces or coils for final processing

Question 16

Foil thickness

Answer 16

Is sheet rolled to extremely thin gauge, less than 0.01mm

Question 17

Describe hot rolling and what it does

Answer 17

In most cases is carried out at elevated temperatures depending on the material. Is first step in the process. Utilises low flow stress at higher temperatures to make large initial reductions in gauge. The brittle, porous, coarse grained structure is replaced by a wrought structure which is more ductile and has a finer grain size.

Question 18

Describe cold rolling and what it does

Answer 18

Is the second stage in the process. Allows good surface finish to be achieved and allows greater dimensional control than in hot rolling

Question 19

Importance of friction and factors that affect it

Answer 19

Without friction rolling would be impossible as the metal wouldn’t be able to pass through the roll bite.
Factors affecting friction are: temperature, material, roll diameter, lubrication

Question 20

Impacts of roll forces and solutions

Answer 20

Roll forces tend to bend the rolls with the result that the strip is thicker at the centre than the edges. Solution is to machine the rolls so that the diameter at the centre is large than at the edge so that when properly designed flat plate or sheet is produced.
Mill spring (rolls being forced apart) means plate or sheet reduced doesn’t have the thickness of the roll gap. Mill spring can be calculated for a particular mill so that correct roll gap settings can be made for the product being rolled

Question 21

What can cause surface defects and how is one of the causes dealt with?

Answer 21

Inclusions in the material, scale, rust or roll markings.
Possible to reduce the defects caused by scale by prior treatment of the ingot

Question 22

Four types of structural defects

Answer 22

Affect the integrity of the product.
Wavy edges or the product not being flat (can solve by pulling from ends).
Edge cracking (if not close to the centre can trim edges).
Centreline cracking or porosity.
Alligatoring (maybe tried to deform it too much at once).
Some of these can be reduced by better control of homogenisation prior to rolling

Question 23

Types of prior treatments and what they are for

Answer 23

Conditioning removes scale prior to rolling and can be done with a flame torch or by rough grinding. Scale produced during hot rolling removed by pickling in acid prior to cold rolling.
Pack rolling where two or more layers of metal rolled together to improve productivity (like Al foil).
Temper rolling alleviates luders band formation in mild steel.
Sheet not sufficiently flat flattened by passing through series of levelling rolls.