Plastic- Case Study 4 Actual Case Study Start Flashcards
Advantages of hot rolling
Measure of microstructural control possible. Restoration processes can be controlled by T, strain rate and strain imparted to material. Deformation loads reduced as high T means low flow stresses. Recovery and RX during hot rolling increase ductility permitting large plastic deformations. High T and large deformations imparted mean any voids formed during casting will be sealed (eliminated) and inclusions in ingot broken up.
Disadvantages of hot rolling
Bad surface finish due to oxidation of material.
Poor dimensional control due to thermal expansion.
The three most important parameters during hot rolling and what they influence
Temperature, strain rate, strain.
Strain rate and temperature influence the flow stress.
Cumulative strain imparted to material will influence recovery and RX and texture
Advantages of cold rolling
Good surface finish. Lubrications gives good friction control.
Good dimensional control.
Enhanced strength due to work hardening.
Texture control
Disadvantages of cold rolling
High deformation loads due to high flow stresses.
Poor ductility limits deformation.
If excessive work hardening occurs then annealing may be required.
Most important parameter during cold rolling and why
Strain.
Basically the flow stress of the material is dependent on the cumulative strain imparted to the material during cold rolling
Stage 1 of rolling schedules
Firstly ingot (over 20tons for steel or 10tons Al) is homogenised
Stage 2 of rolling schedules (description and stats)
Initial rolling carried out on slabbing or reversing mills with 600-1600mm rolls using a series of passes. Coolant (water/mineral) sprayed in roll bite to control temperature. Starting ingot 400mm thick rolled down to 5mm. Normally two-high or three-high mill used. Large reduction in short time desired so large diameter rolls used with grooves cut into them to increase friction coefficient.
How many passes might there be in a typical rolling schedule?
In region of 18
Stage 3 of rolling schedules
Further hot rolling carried out on a tandem mill (several rolling mills in series with material passing through in one direction). Material exits this stage about 2mm thick. Complex process as speed of material increases as it passes through each set of rolls. Coolers at entrance and exit of tandem facilitate handling and can also be used to apply front and back tension to reduce rolling loads
Stage 4 of rolling schedules
Cold rolling then used to reduce thickness down to about 0.025mm for steel sheet and under 0.015mm for Al foil. Cold rolling uses multi stand tandem mill. Lubrication used to control friction/surface finish. Back up rolls provide greater rolling mill rigidity and so provide better gauge control (reduce mill spring). Rolls in contact with metal have smaller diameter to reduce rolling load. Back up rolls have much larger diameter to provide rigidity.
Remit of actual case study 4
Producing 5mm thick Al plate from ingot (6mm +/-0.5 thick and 0.5m wide) using hot and cold rolling stages. Final roll gap is 5mm. Final plate thickness is 5.25 with standard deviation 0.1mm. In past this was fine as customer wanted +/-10%. Now wants +/-1% and current product outside this. Need to suggest modifications to process. Flow stress goes from 140MPa +/- 10 to 160MPa +/-10
Assumption about increasing width
If sheet is relatively wide with respect to thickness (5:1), as is the case for the majority of rolling operations, it can be assumed that no increase in width occurs (plain strain) such that the vertical compression of the stack is translated into an elongation in the rolling direction
Equation of equal volume flow rate through rolling
w.h0.V0=w.hf.Vf
Where w is width of sheet.
V0 is velocity of sheet before rolls and Vf after.
h is sheet thickness before and after rolls
Forced on material from roll and where they are
See slide 15 diagram.
Neutral point N mid way through contact between metal and roll. Before this material flowing slower than roll and faster than roll after N. Friction force Pf is tangential to roll circumference always pointing towards N. Radial force Pr perpendicular to Pf pointing into metal
What is rolling load?
The vertical force component acting on the metal is known as the rolling load (F). This is the force with which the rolls press against the metal and equal to the force exerted by the metal in trying to force the rolls apart. Therefore also referred to as the separating force
Simple formula for rolling load
F=χ.w.L.σflow
Where χ is factor slowing for friction and often taken as 1.2 for cold rolling.
w is width.
L is the projected arc of contact between metal and rolls (see slide 17)
σflow is mean flow stress of material being deformed
Final equation for rolling load subbing in for L
F=χ.w.rt(R(h0-hf)).σflow
Where root term is for L
χ accounts for friction (1.2 for cold roll)
w is width, σflow is average flow stress.
R is roll radius and h0 and hf initial and final thickness
What 6 things increase rolling load?
Increasing flow stress
Increased rolling reduction (h0-hf)
Increased roll radius R
Increased width
Decreasing T increases flow stress so increases rolling load.
Increasing rolling speed increases strain rate so increases rolling rolling load.
Why is friction condition between rolls and material extremely important?
If very low then material will not be drawn into rolls and rolling won’t occur. If friction very high than the loads increase dramatically and edge cracking can occur. By considering forces which act on material whilst being rolled we can ascertain the limiting conditions for unaided entry of slab into the rolls (minimum friction requirements).
