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.