evolution of surface roughness during cold rolling of stainless steel rehan ahmed & michael...
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EVOLUTION OF SURFACE ROUGHNESS DURING COLD ROLLING OF STAINLESS STEELREHAN AHMED & MICHAEL SUTCLIFFEHeriot-Watt University Cambridge UniversityDept. Of Mech.. & Chem.. Eng. Department Of EngineeringUnited Kingdom United Kingdom
Industrial CollaboratorsBritish SteelAvesta Sheffield
INTRODUCTION
• Evolution of surface during cold rolling of stainless steel differs significantly from aluminium rolling in the sense that shot-blasting and acid pickling can induce surface features.
• Rolling schedule is thus optimised to eliminate these features as they influence the sheet performance.
AIMS & OBJECTIVES
• Develop a model to extract surface features such as roll marks and pits on the sheet surface.
• Quantify surface feature changes through the roll bite for typical pass schedule.
• Investigate the influence of surface features on functional requirements of sheet surface.
SEM OBSERVATIONS (FINAL PASS)
Grain Boundaries
Micro-pits
EXPERIMENTAL TEST PROCEDURE
• Mill trials were conducted for a typical pass schedule is a 20-high Sendzimir mill.
• Stainless steel sheet of thickness 4.0 mm was rolled to a final thickness of 1.5 mm using a rolling having a viscosity of 10 cSt at 40oC (approximately).
• Samples were collected from the ends of coil after each pass of rolling.,
PIT ANALYSIS
• Process is then repeated for transverse direction
• A summation of ‘n’ number of data points is initially considered at a pit spanning length ‘L’ on either side of individual data point
(n1+n2+ n3+… nn)
L
(n1+n2+ n3+… nn) PIT
Measurement point
• Pits were identified as enclosed features which were deeper than the neighboring surface.
SURFACE FEATURE IDENTIFICATION - pits
j)h(i,k)h(i k)h(i, n
j)(i,dLjk
nLjk
nLjk
Ljktr
1
1
,,
2
1
h(i,j) is the data matrix; d(i,j) is pit depth; is pixel spacing;
L is the pit width; n is the number of points
Pit Depth Criterion (p) was based upon the following equation:
pR
j)(i,d qr
tr ,
SURFACE FEATURE IDENTIFICATION - roll marks• Roll marks were identified as thin long features which
were deeper or higher then the neighboring areas and extend along the rolling direction.
• Combination of the methodology shown in the pit
analysis equation and the slope analysis (i, j) given below:
)jh(i,j)h(i,
j)(i,1
Roll marks were identified using the following equation:
M j)r(i,
Mi
Mi
r(i, j) is the identification matrix for roll marks
M is the critical length of roll mark
SURFACE OBSERVATIONS
Pickled and Shot-blast surface (SEM) After the Final Pass (SEM)
After an Intermediate Pass (AFM)
SENSITIVITY ANALYSIS
0
5
10
15
20
25
30
35
40
45
0 0.5 1 1.5 2
Tot
al p
it a
rea
(%)
L=35
L=25
L=10
L=5
p
0
20
40
60
80
100
120
0 0.5 1
Rol
l mar
k ar
ea (%
)
Contribution from height criterionContribution from slope criterion
Combined roll mark area
m
(p)
(m)
Effect of pit depth tolerance (p) and spanning length (L) on total pit area
Contribution from depth and slope criteria tolerance (m) on roll mark area
RESULTS - Evolution of strip surface in pass schedule
0
10
20
30
40
50
60
Pass Number
Are
a (%
)
Total pit area
Deep pit
Shallow pit
Roll marks
First Intermediate Final
RESULTS - Surface evolution through the bite
0
5
10
15
20
25
30
0 0.02 0.04 0.06 0.08 0.1 0.12
Strain
Are
a (
%)
Total pit areaDeep pitShallow pitRoll marks
Intermediate pass of rolling
RESULTS - Pit area and surface gloss
0
10
20
30
40
50
Pass number
Glo
ss
(%
)
0
10
20
30
40
50
60
To
tal
pit
are
a (%
)
Gloss
Total pit area
First FinalIntermediate
CONCLUSIONS
• A novel method has been established to identify surface features on cold rolled stainless steel strip.
• Larger features are rolled out in earlier passes of rolling while shallower ones persist.
• Sharp change in surface features at the inlet to roll bite indicate the strong influence of strain change with position.
• Correlation between surface reflectivity and pit analysis demonstrate a bench mark for future modeling.