Plastic- Case Study 4: Gauge Control During Cold Rolling Flashcards
The two groups of deformation processes
Primary working operations: rolling, extrusion, forging.
Secondary working operations: wire drawing, machining.
What is rolling?
The process of reducing the thickness (gauge) or cross section of a workpiece by compressive forces applied through a set of rolls.
Types of rolling
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.
Four considerations when rolling
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?
What are two high and four high mills?
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
Smaller diameter rolls
More preferable because the smaller the radius the lower the roll force. But smaller diameter rolls require back up rolls to increase stability
What types of mills are suitable for cold rolling of high strength materials?
Cluster or sendzimir mills as they provide rigidity
Describe the work roll
Can be very small in diameter and usually made of or coated with tungsten carbide or a similar material
Describe the layout of a cluster mill
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
Describe the Steckel mill
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
What are the requirements for roll materials?
Strength and wear resistance. Means common materials are steel and cast iron
How are rolls modified for hot rolling?
The rolls are rough and have notches or grooves in order to pull the metal through
What is done to rolls for coils rolling?
They are ground to a very fine finish and may even be polished to improve surface quality
Plates thickness and uses
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.
Sheet thickness and uses
Rolled material with thickness less than 6mm. Used for car bodies, packaging, aircraft fuselages. Produced in flat pieces or coils for final processing
Foil thickness
Is sheet rolled to extremely thin gauge, less than 0.01mm
Describe hot rolling and what it does
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.
Describe cold rolling and what it does
Is the second stage in the process. Allows good surface finish to be achieved and allows greater dimensional control than in hot rolling
Importance of friction and factors that affect it
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
Impacts of roll forces and solutions
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
What can cause surface defects and how is one of the causes dealt with?
Inclusions in the material, scale, rust or roll markings.
Possible to reduce the defects caused by scale by prior treatment of the ingot
Four types of structural defects
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
Types of prior treatments and what they are for
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.
