1. ColdRollingTechnology
1.TYPES OF COLD ROLLING MILLS
2. Different Components of PL-TCM
3. Edge Drop, Crown,Wedge
4.Roll Force
5.A.S.C
6.A.G.C
7.Defects
3. ColdRollingTechnology
What is Cold Rolling ?
Process by which hot rolled strip or stock is
introduced between rollers and then squeezed or
compressed to the desired thickness
Amount of strain introduced determines the
properties of the finished product
Why is Cold Rolling done ?
Good Formability
Superior Surface Finish
Reasonable Strength
Close Dimensional Tolerance
4. ColdRollingTechnology
Commonly used Grades of Cold Rolled Steel
D/DD - Drawing/Deep Drawing Simple forming/bending
applications
EDD - Extra Deep Drawing Deep Drawn components
5. ColdRollingTechnology
Commonly used Grades of Cold Rolled Steel
IF (Interstitial Free) Critical Draw and StretchComponents
in terms of geometry of the
component, type and depth of draw
and press parameters
IFHS (Interstitial Free High Strength) For components
involving draw and
requiring resisitance
against dent and
impact
8. ColdRollingTechnology
Pickling (to
remove oxides
and scales)
Cold Rolling in
reversing or
tandem mill to
desired thickness
Annealing
(batch or
continuous)
for stress
relieving
Skinpassing to remove
Luder lines, develop
mech prop,impart desired
surface finish improve
flatness
Process Flow
Inspection,
Finishing
Despatch
11. ColdRollingTechnology
Pickling
Why is Pickling done ?
To remove scales and oxide layers present on the
surface of hot rolled steel
Make the steel surface suitable for cold rolling
Scales and oxide layers are harder than steel and
hence hinder the process of cold rolling of low carbon
steel
12. ColdRollingTechnology
Pickling
How’s scale and oxide removal are achieved?
Hot rolled strip is passed through tanks containing
hydrochloric acid wherein the acid dissolves the
oxide layer
The strip is then rinsed with water to remove any
excess acid. dried and the strip is ready for cold
rolling
13. ColdRollingTechnology
Pickling
Process Variables :
Temperature : Strip temperature of 50 to 80degrees
centigrade is maintained. This ensures
faster rate of reaction with the acid
Acid Concentration: Acid concentration in tanks influence
the dissolution of scales and oxides
Lower concentration is maintained in
the first tank and increases in the
subsequent tanks
Line Speed : Controls the residence time of the strip
in the bath
14. ColdRollingTechnology
Cold Rolling
Hot rolled Pickled Coils are reduced to the desired
thickness
Achieved either in
Reversing Cold Rolling Mills : Thickness achieved by
repeated passes through the
same rolling stand back and
forth
or
Tandem Cold Rolling Mills : Thickness achieved through
series of rolling stands in
tandem
15. ColdRollingTechnology
Cold Rolling (Reversing vis a vis Tandem Mill)
Criteria Reversing Mills Tandem Mills (PLTCM)
Productivity Lower Higher
Efficiency Threading and tailling
off are required for
every coil.
Threading and tailling off
not required for every coil.
Increase in efficiency by 50
to 60%
Yield Lower yield - Off
gauge portion from
head and tail end
needs to be removed.
Higher yield - Off gauge
removal not required end
to end rolling achieved.
Use as combination mill
(reduction & temper
rolling)
Can be used. Can be used only for
reduction rolling
17. ColdRollingTechnology
Tata Steel Capability
5 Stand 6Hi Tandem Cold Mill coupled to a
Continuous Pickling Line (PLTCM)
Features
Automated Shape and Gauge Control
Thickness tol achievable - +/- 0.5% of thickness
along centre line with edge drop of 30 microns
Mill Clean System
20. ColdRollingTechnology
Flatness
Flatness is a basic requirement of cold rolled sheets
Affects
• Assembly Accuracy
• Efficiency of the assembly operation
• Straightness and rigidity for panel applications
• Material movement in high speed transfer press lines
24. ColdRollingTechnology
Measurement of
Flatness
Flatness of cold
rolled strip is measured
in terms of height of
wave and wavelength
and denoted in I units
as
I = (/2 x h/L)2 x 105
where h is the wave height and L is the
length of the wave
30. ColdRollingTechnology
Surface Texture
Pattern imparted on the steel surface by the grit on
the roll
Affects the
Paint film quality of the steel surface.
Image clarity after painting
Forming Operations
Two important criterion of Surface Texture
Surface Roughness : Surface Characteristic with
wavelength <800 m
Surface Waviness : Surface Characteristic with
wavelength >800 m
32. ColdRollingTechnology
Surface Texture
Parametrs used as index of surface roughness
Ra value : Average/mean height of surface peaks
and troughs over a reference length
Indicates an overall profile of the sheet
surface, dullness or brightness
34. ColdRollingTechnology
Surface Texture
Rz Value : Sum of the average value of the peak
heights and deepest valley measured
over 5 peaks and 5 crests along the
reference length.
Rz and Rmax indicate the distribution of
matte/roughness on the strip surface.
35. ColdRollingTechnology
WCA Value : Measure of surface waviness
Lower the WCA value , better is the image clarity
after painting
Preferred WCA value for auto panels should be < or = 0.6 m
36. ColdRollingTechnology
Surface Texture After painting.
Roughness gets covered by the paint
Waviness remains after painting.
Bright Finish - (Ra value < 0.6 m)
Desirable for - Any kind of plating operations which
require smooth surface
Matte Finish - Coating
- Helps in drawing due to oil retaining
capacity
Extent of desirable roughness varies with the type of
painting technology used.
37. ColdRollingTechnology
Surface Texture
Tata Steel’s Capability
Electro Dip Texturizing facility
Features
Uniform distribution of Roughness
Control Surface waviness (WCA < 0.6 m) for
better image clarity of auto panels
40. ColdRollingTechnology
Shape Defects
Shape defects in cold rolled strip generate due to types of
strain developed in cold rolled sheets.
Flat steel sheets are the one with minimum strain
Fundamental cause of strain is non uniform elongation of
steel sheet
Higher elongation at the
edges with respect to the
centre of the sheet
generates edge waviness
42. ColdRollingTechnology
Shape Defects
Camber/Cross bow
Measure of
straightness of edge
Higher thickness
variation in transverse
direction,
inacurracy during
slitting gives rise to
camber or cross bow
Normal limit is 2mm
deviation in 2000mm
length
43. ColdRollingTechnology
Shape Defects
Coil Set or Longitudinal Bow
Skin Passed Coils
tend to
retain its shape
when uncoiled due
to the coiling
tension.
Corrected by
applying load in
straightner during Cut to Length operations
44. ColdRollingTechnology
Surface Defects
Scales
Left over oxides formed during hot rolling
Rolled into the sheet surface remaining
unremoved even after pickling
Slivers
Defect mainly caused by alumina inclusions
entrapped during steel making/casting operations
Gets elongated linearly by rolling
Leads to cracking/failure during drawing operation
45. ColdRollingTechnology
Surface Defects
Roll Marks
Defect transferred to the strip from the rolls in either
convex or concave shape
Appears at regular intervals in the rolling directions
Caused due to scratches on rolls , foreign substance
sticking to rolls,metal pick up on rolls during strip
breakage
46. ColdRollingTechnology
Surface Defects
Sticker Marks
Semi circular marks on the surface of the strip.
Generates due to sticking tendency of subsequent
coil wraps
Sticking tendency is due to any of the following reasons
Higher tension of the coils being taken for annealing
Higher soaking time/temperature
Poor shape of the strip
47. ColdRollingTechnology
Surface Defects
Orange Peel
Appears as wrinkles on steel surface
Occurs - Due to coarse grain size - coarser than
ASTM 6
High and Low Spots
Observed on formed components either as dents
or depression on the surface
Occurs due to dirt from atmosphere or from rust
preventive oil used
High Rmax value also results in high spot/low spot
50. ColdRollingTechnology
Annealing ensures
Softening of the cold rolled steel- Relieving the
internal stress developed during cold rolling
Develop desired mechanical properties suitable
for intended applications
Bright Surface Finish
51. ColdRollingTechnology
Annealing
Stages of Annealing
Process of relieving internal stresses induced during
cold rolling takes place in three stages
1. Recovery
Physical and mechanical properties ( Hardness, ductility
elect prop.)which suffered changes as a result of cold
working tend to recover to their original values.
Occurs at the initial phase of annealing at lower
temperatures
52. ColdRollingTechnology
Annealing
Stages of Annealing
2. Recrystallization
Process of nucleation of new strainfree crystals
(grains) which replaces all deformed crystals of
the cold worked material
Takes place at a temperature known as
recrystallization temperature
53. ColdRollingTechnology
Annealing
3 Grain Growth
Increase in average size of crystals/grains on
further annealing subsequent to the completion
of recrystallization
Grain size has direct effect on the mechanical
properties. Hence it is controlled to achieve
optimum properties
54. ColdRollingTechnology
Annealing
Annealing requires
Heating the cold rolled steel to the recrystallization temperature
in an inert atmosphere.
Soaking at this temperature for sufficient time for homogenization
and necessary changes to occur
Cooling at a predetermined rate
Process Variables
Temperature, soaking time , cooling rate depending on
the desired mechanical properties
55. ColdRollingTechnology
Comparison of Batch and Continuous Annealing
Parameters Batch Annealing Continuous Annealing
Chemistry Suitable for low
Carbon low alloy steel
Best suited for IF Steels
Uniformity of Mechanical
Properties
Variations between
outer/ inner wraps to
centre of the coil
Uniform over the length of
the coil
Productivity Lower Higher
Properties with identical
chemistry other than IF
Lower hardness and
YS
Higher hardness and YS
Higher r Bar value Lower r Bar value
No ageing Ageing
IF Steel Properties Good properties
achievable with longer
cycle time.
Better properties
achieved with much lower
cycle
56. ColdRollingTechnology
Annealing
Tata Steel’s Capability
100% Hydrogen Batch Annealing
Unique Features
Higher Productivity
Uniformity of Properties
Surface Cleanliness- Carbon residue <5mg/m2/side
Iron Fines < 15mg/m2/side
Coil Cooling and Storage (CCSU) in dehumidified
chamber- Prevents rust formation in annealed coils
59. ColdRollingTechnology
Skin Passing
Why is Skin Passing Done ?
To achieve the optimum mechanical properties
(Hardness, YS, )
To prevent occurrence of stretcher strains in
annealed steel
To impart desired surface finish - “matte” on the
strip surface
To improve strip shape
60. ColdRollingTechnology
Skin Passing
What are Stretcher Strains?
Generated due to the yield point phenomenon
exhibited by cold rolled and annealed low carbon steel
sheet/strip
Appears as discrete
bands on the strip
surface
61. ColdRollingTechnology
Skin Passing
Mechanical Properties are
controlled by
regulating the % elongation
during skinpasing
% Elongation depends on the
grade of the CR steel.
Range -
Stretcher Strain removal is
achieved through suppression
of the yield point elongation during
skinpassing
Before Skin Pass with Yield Point
Elongation or Stretcher Strain
After Skin Pass Stretcher Strain Suppressed
62. ColdRollingTechnology
Skin Passing
Tata Steel’s Capability
Single Stand 4 Hi SkinPass Mill
Unique features
SkinPassing with temper fluid -Wet skin passing
- Ensures freedom from dirt/dents, any foreign
particles on strip surface.
66. ColdRollingTechnology
Yield Strength (YS)
Stress required to produce a small specified amount of
plastic/permanent deformation.
Indicative of ease of forming operation.
Higher YS - Higher Load for
onset of plastic
or permanent
deformation
Lower YS - Onset of
permanent
deformation at
lower load. Hence
suitable for critical
draw applications
67. ColdRollingTechnology
Ultimate Tensile Strength (UTS)
Maximum load which the steel can withstand
before fracture/ breaking
Measure of the strength of the
material
YS/UTS ratio
Indicates the workability of steel
Lower ratio (< or =0.5) indicates
better workability
69. ColdRollingTechnology
n- Value : Modulus of Work hardening or strain hardening
exponent
Indicates better stretch formability
Calculated from the formula = Kn where is the
true stress, is true strain and K is the material
constant
For good stretchability n value should be >=0.22
70. ColdRollingTechnology
r & r bar - Value
Quantitative index of the anisotropical property- Directionality
Ratio of strain along width to strain along thickness
Measured along the rolling direction, transverse to the rolling
direction and at 45degrees to the rolling direction indicated as
r1, r90 and r45.
Measured in tensile testing machine or in an instrument module r
r bar value is calculated as r1 + r90 + 2 r45
-----------------
4
71. ColdRollingTechnology
Indicative of drawability and resistance to thinning
during drawing operation.
Higher the value, less susceptibility to fracture,
hence better drawability
r & r bar - Value
72. ColdRollingTechnology
Typical Mechanical Properties of Cold Rolled Close Annealed
Steel (CRCA)
Hardness in
Rockwell B
scale YS in Kg/mm2
UTS in Kg/mm2
%
Elongation
on 50mm
GL
r bar
value
min
n-value
min
Grades
D/DD 48 to 55 >17, < 24 28 to 34 35 min 1.3 0.18
EDD 35 to 45 15 to 18 27 to 32 45 min 1.6 0.2
IF <38 13 to 16 27 to 30 48 min 1.8 >0.22
74. ColdRollingTechnology
Application of Cold Rolled Steel (low Carbon)
Automobile components
Panels for White Goods/Appliances
Packaging (Drums ,Barrels and Containers)
Furniture
Electrical Panels
General Engineering
76. ColdRollingTechnology
Panels for Appliances
Dead Flatness
Close Dimensional Tolerance Thk +/- 0.01mm
Optimum Hardness to ensure rigidity , dent resistance
and ease of bending. Hardness range 105 to 115VPN
Optimum Oiling levels compatible with degreasing facilities
Surface free from any blemishes- free from dents/scratches/
rust spots, rolled inscales/pittings
Surface finish amenable to modern painting technology
77. ColdRollingTechnology
Packaging - Drums, Barrels and Containers
Close thickness tolerance to maximise yield in terms of no of
drums per ton maintaining the min drum wt.
Ease of welding - Resistance welding
Optimum hardness to ensure rigidity, strength and ease of
bending , while forming of drum ends
50 to 60 Rb for Drum body to ensure rigidity and stiffness
45 t0 55 Rb for Drum ends to ensure against cracking during
seaming operations
78. ColdRollingTechnology
Applications Parameters to be
essentially captured in
TDC and Order Attributes
Automobile Components Grade, Ref Specn, Mechanical
Properties , Surface
Characteristics (Ra Rz,Rmax),
Flatness, Oil levels, Type of
Oil,Packing Norms
Appliances
(White Goods) and Panels
Grade, Hardness, Flatness,
Surface Characteristics, Oil
Levels,Thickness, Length
Tolerances,Diagonal
Difference,Packing Norms
Packaging
Drums and Barrels
Thickness Tolerance, Hardness,
Oil type and Oil level
