Cold Rolled Steel
• Introduction to Cold Rolling
• Processing of CRCA
• Properties and Surface Characteristics of
Cold Rolled Steel (CRCA)
• Defects in Cold Rolled Steel
• Applications of CRCA
Cold Rolled Steel
Introduction to Cold Rolling
Cold Rolled Steel
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
Cold Rolled Steel
Commonly used Grades of Cold Rolled Steel
D/DD - Drawing/Deep Drawing Simple forming/bending
applications
EDD - Extra Deep Drawing Deep Drawn components
Cold Rolled Steel
Commonly used Grades of Cold Rolled Steel
IF (Interstitial Free) Critical Draw and Stretch Components
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 resistance
against dent and
impact
Cold Rolled Steel
Chemical Composition
Typical Ranges
%C %M %Si %S %P %Al N, %Ti %Nb
n ppm
D/DD 0.05 0.3 0.02 0 0 0.05 70
EDD </=.04 0.2 0.02 0 0 0.03 50
IF 0.003 0.2 0.015 0 0 0.04 40 0.06 0.01
IFHS 0.004 0.4 0.007 0 0 0.03 40 0.03 0.04
Cold Rolled Steel
Cold Rolling Process
Cold Rolled Steel
Process Flow
Cold Rolling in Annealing
Pickling (to reversing or (batch or
remove oxides tandem mill to continuous)
and scales) desired thickness for stress
relieving
Skin passing to remove
Inspection, Luder lines, develop
Finishing mech prop, impart
Dispatch desired surface finish
improve flatness
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Pickling
Process Variables :
Temperature : Strip temperature of 50 to 55degrees
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
Higher concentration is maintained in
the first tank and decreases in the
subsequent tanks
Line Speed : Controls the residence time of the strip
in the bath
Cold Rolled Steel
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
Cold Rolled Steel
Cold Rolling (Reversing vis a vis Tandem Mill)
Criteria Reversing Mills Tandem Mills (PLTCM)
Productivity Lower Higher
Efficiency Threading and tailling Threading and tailling off
off are required for not required for every coil.
every coil. Increase in efficiency by 50
to 60%
Yield Lower yield - Off Higher yield - Off gauge
gauge portion from removal not required end
head and tail end to end rolling achieved.
needs to be removed.
Use as combination mill Can be used. Can be used only for
(reduction & temper reduction rolling
rolling)
Cold Rolled Steel
Process Control Parameters and Variables
Dimensional Control
Flatness or Shape Control :
% Cold Reduction
Plant Capability
Cold Rolled Steel
5 Stand 6Hi Tandem Cold Mill coupled to a
Continuous Pickling Line (PLTCM)
Features
Automated Shape and Gauge Control
Thickness tool achievable - +/- 0.5% of thickness
along centre line with edge drop of 30 microns
Mill Clean System
Cold Rolled Steel
Dimensional Tolerances
Thickness Tolerance : 0.5% of thickness on centre line
Width Tolerance : -0 / +7 mm (Mill Edge)
: -0 / +0.4 mm (Trim Edge)
: -0 / +0.3 mm (Slit Coils)
Cold Rolled Steel
Annealing
Cold Rolled Steel
Annealing
Why annealing is done?
Steel Coils are hard and brittle after Cold Rolling
Has Elongated grain Structure
Not suitable for end use
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Annealing
Stages of Annealing
2. Recrystallization
Process of nucleation of new strain free crystals
(grains) which replaces all deformed crystals of
the cold worked material
Takes place at a temperature known as
recrystallization temperature
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Comparison of Batch and Continuous Annealing
Parameters Batch Annealing Continuous Annealing
Chemistry Suitable for low Best suited for IF Steels
Carbon low alloy steel
Uniformity of Mechanical Variations between Uniform over the length of
Properties outer/ inner wraps to the coil
centre of the coil
Productivity Lower Higher
Properties with identical Lower hardness and Higher hardness and YS
chemistry other than IF YS
Higher r Bar value Lower r Bar value
No ageing Ageing
IF Steel Properties Good properties Better properties
achievable with longer achieved with much lower
cycle time. cycle
Cold Rolled Steel
Annealing
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
Cold Rolled Steel
Annealing
Steel’s Capability
Annealing Cycle Times (approx) for various grades
Grades Cycle
CQ 640 ºC - 560 ºC
D/ DDQ 700 ºC - 590 ºC
EDD 720 ºC - 680 ºC
IF 720 ºC - 690 ºC
Cold Rolled Steel
Skin Passing
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Before Skin Pass with Yield Point
Skin Passing Elongation or Stretcher Strain
Mechanical Properties are
controlled by
regulating the % elongation
during skin passing
% Elongation depends on the
grade of the CR steel. After Skin Pass Stretcher Strain Suppressed
Range -
Stretcher Strain removal is
achieved through suppression
of the yield point elongation during
skin passing
Cold Rolled Steel
Skin Passings
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.
Cold Rolled Steel
SPM Constraint:
Minimum Width is 930mm.
Coil ID is 508mm. Changeover Time to 608mm is 4 hrs.
Bright Rolling is to be taken only in campaign (Min 300T)
Cold Rolled Steel
Properties and Surface Characteristics of Cold
Rolled Steel (CRCA)
Cold Rolled Steel
Properties of Cold Rolled Steel
Mechanical Properties
Hardness
Yield Strength
Ultimate Tensile Strength
% Elongation
Plastic Strain Ratio
Yield Strength (YS) Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Elongation
Measure of the strain before
fracture.
Indicative of the extent to
which metal can be deformed
without fracture
Cold Rolled Steel
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
Cold Rolled Steel
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
r0, r90 and r45.
Measured in tensile testing machine or in an instrument module r
r bar value is calculated as r0 + r90 + 2 r45
-----------------
4
Cold Rolled Steel
r & r bar - Value
Indicative of drawability and resistance to thinning
during drawing operation.
Higher the value, less susceptibility to fracture,
hence better drawability
Cold Rolled Steel
Typical Mechanical Properties of Cold Rolled Close Annealed
Steel (CRCA)
%
Hardness in Elongation r bar
Rockwell B on 50mm value n-value
scale YS in Kg/mm2UTS in Kg/mm2 GL min 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
Cold Rolled Steel
Aging Phenomenon
Change in the properties of steel with elapsing time.
Phenomenon in which yield strength and tensile
strength increases, yield point elongation or luder
bands reappears and elongation decreases with
elapsing time after skin passing.
Interstitial elements Carbon and Nitrogen are
responsible for this phenomenon. Hence this is not a
problem with IF Steels
For other grades of steel it is suggested to use the
steel within three months of shipment to avoid
aging
Cold Rolled Steel
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
Cold Rolled Steel
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 = (d/2 x h/L)2 x 105
where h is the wave height and L is the
length of the wave
Cold Rolled Steel
Flatness
Steel Capability
Flatness achievable
at PLTCM - is 10 I units
at SkinPass Mill - is 8 I units
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
Rmax: Distance between the highest of profile
peak and bottom most of profile valley
line over a reference length
Cold Rolled Steel
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.
Cold Rolled Steel
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
Cold Rolled Steel
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.
Cold Rolled Steel
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
Cold Rolled Steel
The Specifications (CR)
ASTM A366 / A619 / A620
JIS G3141 SPCC / SPCD / SPCE / SPCEN
EN 10130 FEP01 / FEP02 / FEP03 / FEP04 / FEP05
DIN1623 St 12 / St 13 / St 14
BS 1449 CR4 / CR3 / CR2 / CR1
Cold Rolled Steel
Defects in Cold Rolled Steel
Cold Rolled Steel
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
Cold Rolled Steel
Shape Defects
Higher Elongation at
centre than at the edges
gives rise to centre
buckle or a quarter
buckle depending on the
distance from the edge
and centre of the sheet
Cold Rolled Steel
Shape Defects
Camber/Cross bow
Measure of
straightness of edge
Higher thickness
variation in transverse
direction, Normal limit is 2mm
inacurracy during deviation in 2000mm
slitting gives rise to length
camber or cross bow
Cold Rolled Steel
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
Cold Rolled Steel
Surface Defects
Scales
Left over oxides formed during hot rolling
Rolled into the sheet surface remaining
un removed 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
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
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
Cold Rolled Steel
APPLICATIONS OF CRCA
Cold Rolled Steel
Application of Cold Rolled Steel (low Carbon)
Automobile components
Panels for White Goods/Appliances
Packaging (Drums ,Barrels and Containers)
Furniture
Electrical Panels
General Engineering
Cold Rolled Steel
Automobile Components
Excellent drawability- r bar >1.7
YS/UTS ratio < or = 0.5
Surface finish and texture amenable to modern
painting technology - Ra , Rz and Rmax values
Good Weldability
Corrosion Resistance
Dent Resistance
Cold Rolled Steel
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
Surface free from any blemishes- free from dents/scratches/
rust spots, rolled in scales/pittings
Surface finish amenable to modern painting technology
Cold Rolled Steel
Packaging - Drums, Barrels and Containers
Close thickness tolerance to maximize 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
