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Casting Defects in Slab
Power Point Presentation : Employee Training Centre, Dt: 25/05/2011 (11.00 AM-1230 PM
Copyright 2011 © Tata Steel Ltd., India. All rights reserved.
Pabitra Palai
Flat Product Technology Group (FPTG)
Tata Steel Ltd. Jamshedpur
Module-1
Outlines
 Continuous casting and Definitions
 Casting Defects : An overview
 Solidification of Steel; Numerical Equations
 Role of Constituents on casting characteristics
 Defects from Casting
Sticker
Thin Shell
Crack, MLF, Hook Formation
 Defects in Products
 Surface and Sub Surface Defects
Segregation
Cracks
 Shape Defects in Slab : Causes and Remedial measures
Slide 2
Key Definitions
Continuous Casting ; is the process whereby liq. metal is solidified into a "semi-
finished" billet, bloom, or slab for subsequent rolling in the finishing mills. [Sir
Henry Bessmer-1958]
Casting Defect ; Any unwanted deviation from the customers requirements in
a cast slab during continuous casting results in a defect. Some defects in the
cast products are tolerable while others can be rectified by additional
processes like scarfing etc. The following are the 2 major defects which are
likely to occur in continuous castings:
1. Defects during Process
2. Defect in products/ Metallurgical defects
Slab Casting; The Huge slab casters solidify sections up to 250 mm thick and
2,600 mm wide at production rates of up to three million tons per year.
Sliver; The elongated line type surface defects, termed FeO sliver and found
on rolled coil/sheet. Sliver causes both cosmetic surface imperfection and
forming problem .
Slide 3
Sticker
Thin Shell
Crack
Air entrainment
MLF
Hook Formation
Classification
Internal Defects
Long Bow
Out of shape
Camber
Shape Defect Surface defect
Taper
Internal Cracks
Segregations
CLS
Oscillation Mark
Slag spot
Blow, Pin hole
Longitudinal Crack
Transverse Crack
Rhomboidity Trapezoid
Fin
Depressions
Concavity Bulging
NMI
Bleed
Lap
Slab Defects
Slide 4
Process Defects
Defects in Slab
CASTING DEFECTS
Casting Defect in Slab
Steel Solidification models in Continuous Casting
Slide 6
The solidification models are nonlinear differential equations representing the
conservation of mass, momentum and energy, boundary and initial conditions
which depend on the shape of the slab/ mold, and the cooling system
Fluid flow, Heat transfer, Phase changes, Solid mechanics and
Electromagnetics
One-dimensional heat conduction equation
Where, K is the thermal conductivity (W/m K), c the specific heat (J/kg K), ρ the density
(kg/m3), q is the rate of energy generation (W/m3), T the temperature (K), t the time
(s), and x the rectangular coordinate (m).
The release of latent heat
where L is the latent heat (J/kg), fs the
local solid fraction (%)
The fraction of solid in the mushy zone
where Tf is the melting temperature (K), Tl the liquidus temperature (K), and k0 the
partition coefficient
Shell Growth
Slide 7
Steel shell growth can be predicted using Fick’s law
Slab Length (L) = V x (D/K)2
Where L, slab length in m,
V is the casting speed, m/min
D is shell thickness, m
K is solidification constant depends on steel grade and M/c design
Fig. Temp profile from slab
surface : K. Bruce
Fig. : Calculated and measured solidifying shell
thicknesses [R. Pierer ,BHM, 150. Jg (2005)]
Fig.: The shell thickness vs length from meniscus for Al-
7wt % Si at different conditions of casting speed, (Q= 4
m3/hr, d = 5 cm) [Y. Rihan etal, OJMS’W10-0025].
CE and Ferrite potential
Slide 8
For a multicomponent alloy system effect of various elements is considered in
equivalence to carbon by the formula:
C=%[C]+0.04[%Mn]+0.1[%Ni]+0.7[%N]-0.14[%S]-0.4[%Cr]-
0.1[%Mo]-0.24[%Ti]-0.7[%Si]
Ferrite potential is defined as the ferrite fraction
Fp=2.5(0.5-C)
Fp>1 -hypoperitectic steel
Fp<1 -hyperperitectic steel
Effect of Alloying elements
Sulphur (S):
Effects ductility at two temperature, near solidus and below 1200oC.
At Solidus S~0.03% reduces strength and ductility by half.
At 1200oC liquid sulphides at austenitic grain boundaries affect ductility
If Mn:S>60, steel is not embrittled ,as all S is tied to Mn.
Phosphorous(P):
Loss of ductility due to low MP liquid film in the interdendritic region
Responsible for hot tears
Calcium (Ca):
Mitigating influence on cracking sensitivity
Removes S by CaS formation, speeds up final solidification and reduces
Brittle temp range .Act as nuclei on which MnS precipitates
Aluminum(Al):
Precipitation of AlN at grain boundary in the form of a film reduces ductility
drastically
Cr, Ni: Worsen crack sensitivity
Silicon(Si): Improves crack susceptibility
Slide 9
Continuous Casting and Defects
Slide 10
Fig.: Schematic of continuous casting phenomena in mold [B.G. Thomas ’ 03]
Sticker
Thin Shell
Crack
Air entrainment
MLF
Hook Formation
More than 90% of steel production is through continuous casting route
Online prediction of Casting Defect
Slide 11
Fig.: Defect prediction in continuous casting [Matsushita etal; US Patent’88]
Fig-1
Sticker in Continuous casting
Sticker
Sticker is a casting abnormality during which solidifying slab
sticks to the mold wall and may lead to b/o if no precaution is
taken.
Restricted flow of slag between mold/strand gap leads to loss
of lubrication.
Sticker Appearance on Slab
Fig.: Ripple mark s on slab surface due to sticker B/O, Steel making conference proceedings, 1993
Sticker Formation Mechanism
Inadequate/Insufficient Lubrication
Slag rim Formation
Mold level fluctuation
Biased Flow
Metal Chemistry
Slag Basicity & Fluidity
Fig.: Sticker formation stages, Steel making conference proceedings, 1993
Fig. Characteristic temperature profile history for sticker
breakouts and their detection systems [E. Szekeres’1993].
Fig.: Temperature profile at different thermocouple layer during
sticker B/out
Sticker Detection through BDS
Impact of Sticker
Loss of Productivity
Drastic speed reduction to 0.15m/min
Associated damage to machinery due abrupt speed change
In case of break out
Poor quality Casting
Down grading of slab due to speed change
Ripple mark appearance in S/C requires rework
Metallurgical changes in casting
Cost : (531 Lakhs/Annum)
To reduce the rejection (Last 6 months) from 1.02 % to 0.20 %
Cost incurred on a/c of DG, BO, B/D time
Failure to supply compliance to customers etc.
Precaution to avoid sticker B/O
Casting speed reduced to 0.15 m/min) to heel the slab surface then gradually
revert back to original speed
Local Thin Shell
Local Thin Shell: In the process of continuous casting of steel, sometimes the
shell doesn’t solidify to the required thickness. When shell thinning is limited to a
small area of the strand, it is referred to as Local Thin Shell.
Fig. Schematic of thermal profile in continuous casting mold
Possible reasons :
Biased flow due to SEN port
clogging or alignment can locally
retard shell growth.
Uneven distribution and infiltration
of mould flux between the strand
and the mould wall.
Steel chemistry, Superheat and
solidification behaviour
Mould flux thermal behaviour
Argon flow rate
Casting Speed
Mould condition and life
Temperature distribution in Cu mold
Slide 17
Fig.: Temperature distribution & super heat removal
BG Thomas, IST’06 & Bai’2000
Fig.:Breakout shell thickness profiles and corresponding
model predictions, showing thin shell near location of jet
impingement on narrow face, relative to steady shell growth
down the wide face.27
Solidification and Shrinkage of Steel
Linear expansion coefficient (TLE) & Shrinkage:
1. Phase transformation processes dependent upon the carbon content
2. Large difference in the shrinkage behaviour between the various grades of steel wrt to C
content
3. TLE for Low carbon steel (0.05% C) is calculated to be 21·3 ×10-6 K-1, while the TLE of
0.60% carbon steel is shown to be 19.88 ×10-6 K-1.
[Jhu L.G., IMSM’2007]
There are three distinct stages of shrinkage as molten metals solidify:
1. Liquid shrinkage,
2. Liquid-to-solid shrinkage or Solidification Shrinkage
3. Solid shrinkage-Thermal contraction
* Shrinkage is decided by composition and Liquidus temperature of steel.
Steel Chemistry & Shrinkage
Slide 19
Fig. : Solidification shrinkage curves in the round billets along the height of mould
Mould Taper
SC 104 grade (Ultra Low Carbon ) reported without any LTS in 23 heats cast with 1.2% taper.
In SA413, % of LTS increased with 1.2% taper.
Mould taper
12CrMoV, X42) Mould taper in
(B72LX, ER70S6)
Mould taper (37Mn5V)
Steel Chemistry and Solidification
Temperatures in bottom region of the mould are higher for IF steel grades
because of high liquidus temp. around 1537°C which is around 10°C higher
than peritectic grades. Temperatures of bottom layer T.C.s are around 132°C
at higher speeds.
Temperatures of some T.Cs increased more than others because they are
located at the place where hot metal from SEN impinges on the shell.
Fig.: Liquidus Temp Difference in IF & Peritectic grade – 15 oC approx.
Thermal Cracks
Slide 22
Fig.: Steel chemistry and surface crack in slab [C.Genzano etal. ISS conference’2002]
Look line white lines and in extreme cases looks like sliver . Under SEM it looks as
bas-relief, Presence S on EDAX.
In SA413, % of LTS increased with 1.2% taper.
Temperature variation across Con-cast Mold
Slide 23
Fig. Temperature variation across the Cu mold during continuous casting of Steel
Air Entrainment
Slide 24
Fig.: Optimizing argon gas injection (for a 78-mm bore
nozzle with 90° slide gate). Bai.H ’2000
Fig.: Oxygen content along the slab
centerlines (100-μm inclusions).
1. Inclusions, bubbles, slag and other particles during solidification of steel products is a
critical quality concern leading to rework or rejection of slab
2. Open-stream pouring produces the worst air entrainment problems
3. Leaks, cracks, inadequate sealing between the nozzle joints /porous nozzle material
4. The internal pressure in the nozzle below atmospheric pressure tends to aspirate outside
air and can be identified by nitrogen pickup and dendritic inclusions from reaction in a
high-oxygen environment
Mold Level Fluctuation & Solidification
Slide 25
Fig.: Events during a severe level drop (20 mm for 0.6 second)
that lead to a transverse surface depression.BG Thomas’96
Fig.: Comparison of predicted and measured top surface liquid
levels in steel [Yuan Q, Met. Trans-B’04]
Hook Formation
Slide 26
Hook forms when meniscus freezes and overflow
Associated with oscillation mark and plagues ultra low
C steel grades
Subsurface hook formation leads to slab surface
defect due to entrapment of mold flux and inclusion
laden gas bubbles up to 3 mm from slab s/c.
Hook is of 3 types and shape is affected by thermal
distortion and other mechanisms.
Hook formation is greatly affected by steel grade,
super heat, MLF and oscillation conditions
Fig.: Typical hook shape (right) and comparison of hook-shell
thickness with shell thickness prediction from CON1D
(left).[Shin HJ, TMS’04]
SURFACE DEFECTS
Casting defect in steel slab
Slide 28
Surface and Sub surface Defects
Crack Formation Mechanism
Steel with carbon level of 0.08-0.14% is susceptible to cracks due to peritectic
reaction at ~1500 C.
 Peritectic reaction: δ-Fe + liquid = γ-Fe
 Density of γ -Fe is higher than δ -Fe so shrinkage will occur during solidification
 Shrinkage in shell leads to non uniform shell formation.
 In mold, phase during peritectic reaction makes grain coarse and plasticity of
shell reduces.
 Difference in thermal shrinkage between ∂ and Fe leads to tensile strain/stress
in shell which along with uneven heat transfer in the transverse direction leads
to uneven shell thickness in transverse direction.
 Above stress is relieved by crack formation.
Slide 30
Remedies of Cracks
The usually adopted strategy is reduction of the stresses by keeping the thickness
of the shell to a minimum. This is achieved by reducing the horizontal heat transfer.
Reduction in heat transfer is achieved by:
Proper Mould Powder selection: -increase thickness of the solid layer of slag-
crystallinity of the solid slag layer. Crystalline powder reduces conductivity of
heat-high basicity of powder increases crystallinity and increases softening
point.
Mould Oscillation:-Increase of oscillation frequency of mould results in better
larger infiltrated slag thickness, thus less cracks.
Proper Mould Design: -Multi taper mould to accommodate solidification
shrinkage thus reducing air gaps-Grooved mould surface: results in reduction of
heat transfer at meniscus.-high basicity of powder increases crystallinity and
increases softening point.
Proper SEN Design:-to reduce mold level fluctuations
Longitudinal Facial Crack (LFC)
• Appear in the off-mid portion of slab
• It occurs on the slab surface parallel to casting direction
• Length may vary extend up to several centimetres
• Depth can vary from 3 to 5mm and can go as deep as 15cms!
• Width from 0.2mm to over 5mm.
• Occurrence of LFC varies from 5-20% in cast slab depending on grade.
Effect of LFC
LFC visible on slab surface appears as
• Light single line and sometimes multiple line type sliver on product
• Needs inspection of slabs and repairing, sometime may lead to downgrading
• Lowers slab availability
• There are reports of sliver from LFC
Slide 32
Effect of LFC
Repair of LFC and Sliver Analysis
Fig. Met Lab Analysis of Sliver from Longitudinal Crack
Fig.: Longitudinal Crack repairing
Slide 33
Transverse Crack
Mechanism
Generated by longitudinal tensile strains at the surface in the mould due to
high friction or in the sub mould during straitening or bending.
Cause
 Stress generation in the depressed portion of oscillation mark. Often
found along deep oscillation mark.
 Uneven variation of heat flux in the longitudinal direction due to
improper mould powder
 Excessive mold taper.
 Mold level fluctuation
 Steel composition
 Remedy
 Proper choice of flux and higher flux consumption.
 Proper mold taper.
 Control of mold level fluctuation
 Proper uniform cooling to avoid the low ductility temperature range.
Slide 34
Fig.: Propagation of transverse crack during hot rolling
Effect of Transverse Crack
Slide 35
Star Crack
Star cracks appear in star pattern on the slab surface. It can be single or in
cluster (spider web type). These are visible only after light scarfing i.e., after
removal of 2-3mm of surface.
Mechanism
• Preferential concentration of elements such as Cu on the surface of the
strand due to pick up from the mould.
• Embrittlement of the austenite grain boundary due to liquid copper in
conjunction with tensile stresses.
Cause
• High Cu levels (>0.15%)
Remedies
• Coating of mould walls
• Adjust mould alignment
Slide 36
Internal Cracks in Slab
Slide 37
Segregation
Steel Composition
Improper Cooling
Super heat
Mid -Way Cracks
Cause
Surface Re-heating in or below the spray chamber
High Superheat
S & P > 0.02 %
Remedy
Adjust spray system to minimize re-heating
Lower Superheat
Lower P & S
Internal Cracks
Segregation is more at higher superheat
Slide 38
Triple -Point Cracks
Causes
Bulging of Wide Portion of the slabs
Decreasing Mn< 0.9%
Decreasing Mn/S < 30
Remedies
Re-gap rolls
Centre-Line Cracks
Causes
Bulging of Wide Face
Spray Water Intensity
Low roll alignment in the strand
Remedies
Re-gap Rolls
Reduce Casting Speed
Increase Spray Cooling
Internal Cracks
Slide 39
Diagonal Cracks
Causes
Asymmetric cooling in mould and sprays
High Superheat
Predominant in billets
Remedies
Install Corner rolls at the bottom of the mould
Check alignment between mould and roller apron
Look for plugged nozzles
Internal Cracks
SHAPE DEFECTS
Casting defect in steel slab
Definition
It is the distortion of the slab giving rise to concave surface/surfaces. This
occurs usually due to inadequate support of the skin against the effects of
Ferro static pressure.
Causes
High casting speed
High casting superheat and high mold temperature
Improper mold taper leading to poor cooling
Inadequate roll pressure
High teeming rate and biased flow
Inadequate mold design
Distortion or wear of mold
Fast or uneven cooling in the mold as well as secondary cooling
Bending or straightening of slab at too high or too low temperature
Bulging
Definition
A convex shape that narrows toward another end of the slab. It is generally
observed in slab due to online width change and some times due to improper
mold design or cooling pattern. Optimum taper in mold is provided in order to
compensate the ingot shrinkage.
Causes
Online width changes made by the continuous caster
Remedies
Selection of optimum width change technology
Proper width measurement
Taper
'W1-W2 ≤ 15mm
Rhomboidity
Definition
Cause
• Mold deformation
• Different water gap between Cu tube and water jacket for opposite or adjacent faces
• Mold water quality
• Too high casting temperature
• Susceptibility to a rhomboidal shape is enhanced in the case of high carbon content,
peritectic grades and high level of tramp elements ( S,P,Cu,Sn)
• Bad centering of casting stream in the mold
• Inadequate mold taper
• Misalignment in mold / foot rolls area
Rhomboidity is a shape defect mainly appears in slabs and billets when one diagonal of
the rectangular section is longer than other one. This is formed due to asymmetrical
cooling of the strand. Reduction in shell thickness at meniscus level may reduce
rhomboidity due to equal stress development at all 4 sides, It can be reduced, by
reduction in intermittent heating at any face inside the mold . Flushing can improve the
even cooling all the 4 sides
• Mold control and changing if necessary
• Mold measurement
• Reduction in mold water quantity and adequate water gap
• Correct alignment of foot rolls
• Use of MEMS
• Flushing of the strands
• Temperature according to casting speed
• Increasing relative casting speed
•
The reason behind the above point a) and b) are to reduce the shell thickness at
meniscus level so, the stress developed at all 4 sides become even, which is the
main reason for rhomboidity, can be reduced. Also , by doing this we can reduce
the intermittent heating at any face inside the mold . Flushing can improve the
even cooling all the 4 sides.
Remedies of Rhomboidity
Long Bow
Long Bow
L
LB
LB ≤ 0.01×L
Definition
The long bow is a one piece design and occurs when the split roll bearing are
offset from roll to roll they are inline down the whole of the casting bow. The
reduced heat extraction at the bearing position resulted the heat variation in
stripped pattern.
Causes
Uneven temperature distribution across the slab width
Slab surface temperature variation 100 oC approx. which is visible as stripes on the
strand surface.
Peaks in temp coincided with the split roll bearing positions
Reduced water density in the non optimal spray overlap area
Remedies
 Monitoring the Slab surface temp variation on the casting bow before entry into
straightener
 Ideal nozzle positions and spray heights
 Adequate heat removal from roll bearing area and intensify the strand surface
temp. deviation.
 Support at Centre , Both ends and Three points of the slab
Ray Boyle’2004
Out of shape
Definition
Causes
S ≤ 0.01×W
 When the axes of both edging rolls are perpendicular to the advancing direction of
the slab, the slab tends to ascend on one side becoming tilted. slab geometry
resulted is called as out of shape or out of square cross section which is shown in fig.
1. Also shown a typical out of shape defect in Fig.-2.
 The main cause of an out-of-square slab cross-section is the ascension of one side
of the slab. slab Distortion takes place either due to temperature gradients,
loading or a combination thereof.
Kokubo et al
Remedies
 Improper slab cutting /problem in TCM
 Differences in temperature between the top and bottom surfaces of the slab will
cause the slab to curl. Since slab weight and contact with the base restrict its
movement, stresses are created.
 Load stress. Loads on a slab will create both compressive and tensile stresses
within the slab.
 Shrinkage and expansion. In addition to curling, environmental temperatures
will cause PCC slabs to expand (when hot) and contract (when cool), which
causes change of slab shape
 This is generally prevented by using either grooved rolls or tapered rolls with a
bottom collar.
 To prevent this ascension, the edging roll at this side is tilted at the angle q
toward the same direction as the advancing direction of the slab. For a
reversing pass, the edging roll is tilted at the angel q0. This action creates a
component of the roll tangential force that pushes the slab down against the
table rolls.
Trapezoid Shape
Definition
Causes
ED
T
TRAPEZOID
'2ED≤ 20mm
Trapezoid shape defect mainly appears in slabs when width at one surface is larger
than other one. This is takes place due to slab withdrawal through curved path by
curvature and straightening of slab with the liquid core. This results difference in
guiding the curved and subsequent straightened slab between the two wide side of
the same slab. As a result the end surface of the slab were inclined relative to each
other rather being parallel which is called as the trapezoid shape. This is also formed
due to asymmetrical cooling of the strand.
• Mold deformation to trapezoidal shape in curved mold casting
• Inadequate mold and machine design
• Inadequate mold guide and slab acquiring curvature from the mold
• Misalignment in mold / foot rolls area
Remedies
• Proper design of continuous casting machine and mold.
• The mold having main wide wall disposed on the side of the center of curvature of
the slab, which curvature the slab will assume after leaving the mold.
• Proper mold guide through and out of curvature or making the mold wide wall larger
than the wall opposite to.
Fins
It is the thin strip of metal protruding approximately at right angle to the
surface of the slab. It is caused by molten steel having run into the open
cracks in the mold.
Causes
• Gap between the copper plate in mold
• High casting superheat
• Loosened/untight bolt during mold assembly
• Prolong use of mold
• Inadequate mold vibration
Remedies
• Flame cutting.
• Roll edging.
• Pressing
Concavity
Causes
It is the distortion of the slab which appears as a concave surface. This
defect is usually attributed due to improper spray cooling pattern.
• Improper secondary cooling
• High casting superheat
• Improper mold shape
• Inadequate roll pressure and roll surface profile
• Cooling water quality and volume
Remedies
• Avoid the reasons of the defect
• And flatten the slab by scarfing
Depressions
Definition
Causes
Depression are mainly 2 types. Longitudinal depression is a channel
shaped depression on the face of the slab running in the direction of the
axis. Where as transverse depression is a localized depression on the
slab surface normal to the axis of the slab.
• Uneven lubrication
• Very rapid cooling
• Low casting superheat
• Mold level fluctuation
Remedy
Longitudinal depression
Transverse depression
• Avoid the reasons of the defect
Thank You
Slide 55
DISCUSSIONS

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Casting-Defect-inSlab continuous casting.pdf

  • 1. Casting Defects in Slab Power Point Presentation : Employee Training Centre, Dt: 25/05/2011 (11.00 AM-1230 PM Copyright 2011 © Tata Steel Ltd., India. All rights reserved. Pabitra Palai Flat Product Technology Group (FPTG) Tata Steel Ltd. Jamshedpur Module-1
  • 2. Outlines  Continuous casting and Definitions  Casting Defects : An overview  Solidification of Steel; Numerical Equations  Role of Constituents on casting characteristics  Defects from Casting Sticker Thin Shell Crack, MLF, Hook Formation  Defects in Products  Surface and Sub Surface Defects Segregation Cracks  Shape Defects in Slab : Causes and Remedial measures Slide 2
  • 3. Key Definitions Continuous Casting ; is the process whereby liq. metal is solidified into a "semi- finished" billet, bloom, or slab for subsequent rolling in the finishing mills. [Sir Henry Bessmer-1958] Casting Defect ; Any unwanted deviation from the customers requirements in a cast slab during continuous casting results in a defect. Some defects in the cast products are tolerable while others can be rectified by additional processes like scarfing etc. The following are the 2 major defects which are likely to occur in continuous castings: 1. Defects during Process 2. Defect in products/ Metallurgical defects Slab Casting; The Huge slab casters solidify sections up to 250 mm thick and 2,600 mm wide at production rates of up to three million tons per year. Sliver; The elongated line type surface defects, termed FeO sliver and found on rolled coil/sheet. Sliver causes both cosmetic surface imperfection and forming problem . Slide 3
  • 4. Sticker Thin Shell Crack Air entrainment MLF Hook Formation Classification Internal Defects Long Bow Out of shape Camber Shape Defect Surface defect Taper Internal Cracks Segregations CLS Oscillation Mark Slag spot Blow, Pin hole Longitudinal Crack Transverse Crack Rhomboidity Trapezoid Fin Depressions Concavity Bulging NMI Bleed Lap Slab Defects Slide 4 Process Defects Defects in Slab
  • 6. Steel Solidification models in Continuous Casting Slide 6 The solidification models are nonlinear differential equations representing the conservation of mass, momentum and energy, boundary and initial conditions which depend on the shape of the slab/ mold, and the cooling system Fluid flow, Heat transfer, Phase changes, Solid mechanics and Electromagnetics One-dimensional heat conduction equation Where, K is the thermal conductivity (W/m K), c the specific heat (J/kg K), ρ the density (kg/m3), q is the rate of energy generation (W/m3), T the temperature (K), t the time (s), and x the rectangular coordinate (m). The release of latent heat where L is the latent heat (J/kg), fs the local solid fraction (%) The fraction of solid in the mushy zone where Tf is the melting temperature (K), Tl the liquidus temperature (K), and k0 the partition coefficient
  • 7. Shell Growth Slide 7 Steel shell growth can be predicted using Fick’s law Slab Length (L) = V x (D/K)2 Where L, slab length in m, V is the casting speed, m/min D is shell thickness, m K is solidification constant depends on steel grade and M/c design Fig. Temp profile from slab surface : K. Bruce Fig. : Calculated and measured solidifying shell thicknesses [R. Pierer ,BHM, 150. Jg (2005)] Fig.: The shell thickness vs length from meniscus for Al- 7wt % Si at different conditions of casting speed, (Q= 4 m3/hr, d = 5 cm) [Y. Rihan etal, OJMS’W10-0025].
  • 8. CE and Ferrite potential Slide 8 For a multicomponent alloy system effect of various elements is considered in equivalence to carbon by the formula: C=%[C]+0.04[%Mn]+0.1[%Ni]+0.7[%N]-0.14[%S]-0.4[%Cr]- 0.1[%Mo]-0.24[%Ti]-0.7[%Si] Ferrite potential is defined as the ferrite fraction Fp=2.5(0.5-C) Fp>1 -hypoperitectic steel Fp<1 -hyperperitectic steel
  • 9. Effect of Alloying elements Sulphur (S): Effects ductility at two temperature, near solidus and below 1200oC. At Solidus S~0.03% reduces strength and ductility by half. At 1200oC liquid sulphides at austenitic grain boundaries affect ductility If Mn:S>60, steel is not embrittled ,as all S is tied to Mn. Phosphorous(P): Loss of ductility due to low MP liquid film in the interdendritic region Responsible for hot tears Calcium (Ca): Mitigating influence on cracking sensitivity Removes S by CaS formation, speeds up final solidification and reduces Brittle temp range .Act as nuclei on which MnS precipitates Aluminum(Al): Precipitation of AlN at grain boundary in the form of a film reduces ductility drastically Cr, Ni: Worsen crack sensitivity Silicon(Si): Improves crack susceptibility Slide 9
  • 10. Continuous Casting and Defects Slide 10 Fig.: Schematic of continuous casting phenomena in mold [B.G. Thomas ’ 03] Sticker Thin Shell Crack Air entrainment MLF Hook Formation More than 90% of steel production is through continuous casting route
  • 11. Online prediction of Casting Defect Slide 11 Fig.: Defect prediction in continuous casting [Matsushita etal; US Patent’88] Fig-1
  • 12. Sticker in Continuous casting Sticker Sticker is a casting abnormality during which solidifying slab sticks to the mold wall and may lead to b/o if no precaution is taken. Restricted flow of slag between mold/strand gap leads to loss of lubrication. Sticker Appearance on Slab Fig.: Ripple mark s on slab surface due to sticker B/O, Steel making conference proceedings, 1993
  • 13. Sticker Formation Mechanism Inadequate/Insufficient Lubrication Slag rim Formation Mold level fluctuation Biased Flow Metal Chemistry Slag Basicity & Fluidity Fig.: Sticker formation stages, Steel making conference proceedings, 1993
  • 14. Fig. Characteristic temperature profile history for sticker breakouts and their detection systems [E. Szekeres’1993]. Fig.: Temperature profile at different thermocouple layer during sticker B/out Sticker Detection through BDS
  • 15. Impact of Sticker Loss of Productivity Drastic speed reduction to 0.15m/min Associated damage to machinery due abrupt speed change In case of break out Poor quality Casting Down grading of slab due to speed change Ripple mark appearance in S/C requires rework Metallurgical changes in casting Cost : (531 Lakhs/Annum) To reduce the rejection (Last 6 months) from 1.02 % to 0.20 % Cost incurred on a/c of DG, BO, B/D time Failure to supply compliance to customers etc. Precaution to avoid sticker B/O Casting speed reduced to 0.15 m/min) to heel the slab surface then gradually revert back to original speed
  • 16. Local Thin Shell Local Thin Shell: In the process of continuous casting of steel, sometimes the shell doesn’t solidify to the required thickness. When shell thinning is limited to a small area of the strand, it is referred to as Local Thin Shell. Fig. Schematic of thermal profile in continuous casting mold Possible reasons : Biased flow due to SEN port clogging or alignment can locally retard shell growth. Uneven distribution and infiltration of mould flux between the strand and the mould wall. Steel chemistry, Superheat and solidification behaviour Mould flux thermal behaviour Argon flow rate Casting Speed Mould condition and life
  • 17. Temperature distribution in Cu mold Slide 17 Fig.: Temperature distribution & super heat removal BG Thomas, IST’06 & Bai’2000 Fig.:Breakout shell thickness profiles and corresponding model predictions, showing thin shell near location of jet impingement on narrow face, relative to steady shell growth down the wide face.27
  • 18. Solidification and Shrinkage of Steel Linear expansion coefficient (TLE) & Shrinkage: 1. Phase transformation processes dependent upon the carbon content 2. Large difference in the shrinkage behaviour between the various grades of steel wrt to C content 3. TLE for Low carbon steel (0.05% C) is calculated to be 21·3 ×10-6 K-1, while the TLE of 0.60% carbon steel is shown to be 19.88 ×10-6 K-1. [Jhu L.G., IMSM’2007] There are three distinct stages of shrinkage as molten metals solidify: 1. Liquid shrinkage, 2. Liquid-to-solid shrinkage or Solidification Shrinkage 3. Solid shrinkage-Thermal contraction * Shrinkage is decided by composition and Liquidus temperature of steel.
  • 19. Steel Chemistry & Shrinkage Slide 19 Fig. : Solidification shrinkage curves in the round billets along the height of mould
  • 20. Mould Taper SC 104 grade (Ultra Low Carbon ) reported without any LTS in 23 heats cast with 1.2% taper. In SA413, % of LTS increased with 1.2% taper. Mould taper 12CrMoV, X42) Mould taper in (B72LX, ER70S6) Mould taper (37Mn5V)
  • 21. Steel Chemistry and Solidification Temperatures in bottom region of the mould are higher for IF steel grades because of high liquidus temp. around 1537°C which is around 10°C higher than peritectic grades. Temperatures of bottom layer T.C.s are around 132°C at higher speeds. Temperatures of some T.Cs increased more than others because they are located at the place where hot metal from SEN impinges on the shell. Fig.: Liquidus Temp Difference in IF & Peritectic grade – 15 oC approx.
  • 22. Thermal Cracks Slide 22 Fig.: Steel chemistry and surface crack in slab [C.Genzano etal. ISS conference’2002] Look line white lines and in extreme cases looks like sliver . Under SEM it looks as bas-relief, Presence S on EDAX. In SA413, % of LTS increased with 1.2% taper.
  • 23. Temperature variation across Con-cast Mold Slide 23 Fig. Temperature variation across the Cu mold during continuous casting of Steel
  • 24. Air Entrainment Slide 24 Fig.: Optimizing argon gas injection (for a 78-mm bore nozzle with 90° slide gate). Bai.H ’2000 Fig.: Oxygen content along the slab centerlines (100-μm inclusions). 1. Inclusions, bubbles, slag and other particles during solidification of steel products is a critical quality concern leading to rework or rejection of slab 2. Open-stream pouring produces the worst air entrainment problems 3. Leaks, cracks, inadequate sealing between the nozzle joints /porous nozzle material 4. The internal pressure in the nozzle below atmospheric pressure tends to aspirate outside air and can be identified by nitrogen pickup and dendritic inclusions from reaction in a high-oxygen environment
  • 25. Mold Level Fluctuation & Solidification Slide 25 Fig.: Events during a severe level drop (20 mm for 0.6 second) that lead to a transverse surface depression.BG Thomas’96 Fig.: Comparison of predicted and measured top surface liquid levels in steel [Yuan Q, Met. Trans-B’04]
  • 26. Hook Formation Slide 26 Hook forms when meniscus freezes and overflow Associated with oscillation mark and plagues ultra low C steel grades Subsurface hook formation leads to slab surface defect due to entrapment of mold flux and inclusion laden gas bubbles up to 3 mm from slab s/c. Hook is of 3 types and shape is affected by thermal distortion and other mechanisms. Hook formation is greatly affected by steel grade, super heat, MLF and oscillation conditions Fig.: Typical hook shape (right) and comparison of hook-shell thickness with shell thickness prediction from CON1D (left).[Shin HJ, TMS’04]
  • 28. Slide 28 Surface and Sub surface Defects
  • 29. Crack Formation Mechanism Steel with carbon level of 0.08-0.14% is susceptible to cracks due to peritectic reaction at ~1500 C.  Peritectic reaction: δ-Fe + liquid = γ-Fe  Density of γ -Fe is higher than δ -Fe so shrinkage will occur during solidification  Shrinkage in shell leads to non uniform shell formation.  In mold, phase during peritectic reaction makes grain coarse and plasticity of shell reduces.  Difference in thermal shrinkage between ∂ and Fe leads to tensile strain/stress in shell which along with uneven heat transfer in the transverse direction leads to uneven shell thickness in transverse direction.  Above stress is relieved by crack formation.
  • 30. Slide 30 Remedies of Cracks The usually adopted strategy is reduction of the stresses by keeping the thickness of the shell to a minimum. This is achieved by reducing the horizontal heat transfer. Reduction in heat transfer is achieved by: Proper Mould Powder selection: -increase thickness of the solid layer of slag- crystallinity of the solid slag layer. Crystalline powder reduces conductivity of heat-high basicity of powder increases crystallinity and increases softening point. Mould Oscillation:-Increase of oscillation frequency of mould results in better larger infiltrated slag thickness, thus less cracks. Proper Mould Design: -Multi taper mould to accommodate solidification shrinkage thus reducing air gaps-Grooved mould surface: results in reduction of heat transfer at meniscus.-high basicity of powder increases crystallinity and increases softening point. Proper SEN Design:-to reduce mold level fluctuations
  • 31. Longitudinal Facial Crack (LFC) • Appear in the off-mid portion of slab • It occurs on the slab surface parallel to casting direction • Length may vary extend up to several centimetres • Depth can vary from 3 to 5mm and can go as deep as 15cms! • Width from 0.2mm to over 5mm. • Occurrence of LFC varies from 5-20% in cast slab depending on grade. Effect of LFC LFC visible on slab surface appears as • Light single line and sometimes multiple line type sliver on product • Needs inspection of slabs and repairing, sometime may lead to downgrading • Lowers slab availability • There are reports of sliver from LFC
  • 32. Slide 32 Effect of LFC Repair of LFC and Sliver Analysis Fig. Met Lab Analysis of Sliver from Longitudinal Crack Fig.: Longitudinal Crack repairing
  • 33. Slide 33 Transverse Crack Mechanism Generated by longitudinal tensile strains at the surface in the mould due to high friction or in the sub mould during straitening or bending. Cause  Stress generation in the depressed portion of oscillation mark. Often found along deep oscillation mark.  Uneven variation of heat flux in the longitudinal direction due to improper mould powder  Excessive mold taper.  Mold level fluctuation  Steel composition  Remedy  Proper choice of flux and higher flux consumption.  Proper mold taper.  Control of mold level fluctuation  Proper uniform cooling to avoid the low ductility temperature range.
  • 34. Slide 34 Fig.: Propagation of transverse crack during hot rolling Effect of Transverse Crack
  • 35. Slide 35 Star Crack Star cracks appear in star pattern on the slab surface. It can be single or in cluster (spider web type). These are visible only after light scarfing i.e., after removal of 2-3mm of surface. Mechanism • Preferential concentration of elements such as Cu on the surface of the strand due to pick up from the mould. • Embrittlement of the austenite grain boundary due to liquid copper in conjunction with tensile stresses. Cause • High Cu levels (>0.15%) Remedies • Coating of mould walls • Adjust mould alignment
  • 37. Slide 37 Segregation Steel Composition Improper Cooling Super heat Mid -Way Cracks Cause Surface Re-heating in or below the spray chamber High Superheat S & P > 0.02 % Remedy Adjust spray system to minimize re-heating Lower Superheat Lower P & S Internal Cracks Segregation is more at higher superheat
  • 38. Slide 38 Triple -Point Cracks Causes Bulging of Wide Portion of the slabs Decreasing Mn< 0.9% Decreasing Mn/S < 30 Remedies Re-gap rolls Centre-Line Cracks Causes Bulging of Wide Face Spray Water Intensity Low roll alignment in the strand Remedies Re-gap Rolls Reduce Casting Speed Increase Spray Cooling Internal Cracks
  • 39. Slide 39 Diagonal Cracks Causes Asymmetric cooling in mould and sprays High Superheat Predominant in billets Remedies Install Corner rolls at the bottom of the mould Check alignment between mould and roller apron Look for plugged nozzles Internal Cracks
  • 41. Definition It is the distortion of the slab giving rise to concave surface/surfaces. This occurs usually due to inadequate support of the skin against the effects of Ferro static pressure. Causes High casting speed High casting superheat and high mold temperature Improper mold taper leading to poor cooling Inadequate roll pressure High teeming rate and biased flow Inadequate mold design Distortion or wear of mold Fast or uneven cooling in the mold as well as secondary cooling Bending or straightening of slab at too high or too low temperature Bulging
  • 42. Definition A convex shape that narrows toward another end of the slab. It is generally observed in slab due to online width change and some times due to improper mold design or cooling pattern. Optimum taper in mold is provided in order to compensate the ingot shrinkage. Causes Online width changes made by the continuous caster Remedies Selection of optimum width change technology Proper width measurement Taper 'W1-W2 ≤ 15mm
  • 43. Rhomboidity Definition Cause • Mold deformation • Different water gap between Cu tube and water jacket for opposite or adjacent faces • Mold water quality • Too high casting temperature • Susceptibility to a rhomboidal shape is enhanced in the case of high carbon content, peritectic grades and high level of tramp elements ( S,P,Cu,Sn) • Bad centering of casting stream in the mold • Inadequate mold taper • Misalignment in mold / foot rolls area Rhomboidity is a shape defect mainly appears in slabs and billets when one diagonal of the rectangular section is longer than other one. This is formed due to asymmetrical cooling of the strand. Reduction in shell thickness at meniscus level may reduce rhomboidity due to equal stress development at all 4 sides, It can be reduced, by reduction in intermittent heating at any face inside the mold . Flushing can improve the even cooling all the 4 sides
  • 44. • Mold control and changing if necessary • Mold measurement • Reduction in mold water quantity and adequate water gap • Correct alignment of foot rolls • Use of MEMS • Flushing of the strands • Temperature according to casting speed • Increasing relative casting speed • The reason behind the above point a) and b) are to reduce the shell thickness at meniscus level so, the stress developed at all 4 sides become even, which is the main reason for rhomboidity, can be reduced. Also , by doing this we can reduce the intermittent heating at any face inside the mold . Flushing can improve the even cooling all the 4 sides. Remedies of Rhomboidity
  • 45. Long Bow Long Bow L LB LB ≤ 0.01×L Definition The long bow is a one piece design and occurs when the split roll bearing are offset from roll to roll they are inline down the whole of the casting bow. The reduced heat extraction at the bearing position resulted the heat variation in stripped pattern. Causes Uneven temperature distribution across the slab width Slab surface temperature variation 100 oC approx. which is visible as stripes on the strand surface. Peaks in temp coincided with the split roll bearing positions Reduced water density in the non optimal spray overlap area
  • 46. Remedies  Monitoring the Slab surface temp variation on the casting bow before entry into straightener  Ideal nozzle positions and spray heights  Adequate heat removal from roll bearing area and intensify the strand surface temp. deviation.  Support at Centre , Both ends and Three points of the slab Ray Boyle’2004
  • 47. Out of shape Definition Causes S ≤ 0.01×W  When the axes of both edging rolls are perpendicular to the advancing direction of the slab, the slab tends to ascend on one side becoming tilted. slab geometry resulted is called as out of shape or out of square cross section which is shown in fig. 1. Also shown a typical out of shape defect in Fig.-2.  The main cause of an out-of-square slab cross-section is the ascension of one side of the slab. slab Distortion takes place either due to temperature gradients, loading or a combination thereof. Kokubo et al
  • 48. Remedies  Improper slab cutting /problem in TCM  Differences in temperature between the top and bottom surfaces of the slab will cause the slab to curl. Since slab weight and contact with the base restrict its movement, stresses are created.  Load stress. Loads on a slab will create both compressive and tensile stresses within the slab.  Shrinkage and expansion. In addition to curling, environmental temperatures will cause PCC slabs to expand (when hot) and contract (when cool), which causes change of slab shape  This is generally prevented by using either grooved rolls or tapered rolls with a bottom collar.  To prevent this ascension, the edging roll at this side is tilted at the angle q toward the same direction as the advancing direction of the slab. For a reversing pass, the edging roll is tilted at the angel q0. This action creates a component of the roll tangential force that pushes the slab down against the table rolls.
  • 49. Trapezoid Shape Definition Causes ED T TRAPEZOID '2ED≤ 20mm Trapezoid shape defect mainly appears in slabs when width at one surface is larger than other one. This is takes place due to slab withdrawal through curved path by curvature and straightening of slab with the liquid core. This results difference in guiding the curved and subsequent straightened slab between the two wide side of the same slab. As a result the end surface of the slab were inclined relative to each other rather being parallel which is called as the trapezoid shape. This is also formed due to asymmetrical cooling of the strand. • Mold deformation to trapezoidal shape in curved mold casting • Inadequate mold and machine design • Inadequate mold guide and slab acquiring curvature from the mold • Misalignment in mold / foot rolls area
  • 50. Remedies • Proper design of continuous casting machine and mold. • The mold having main wide wall disposed on the side of the center of curvature of the slab, which curvature the slab will assume after leaving the mold. • Proper mold guide through and out of curvature or making the mold wide wall larger than the wall opposite to.
  • 51. Fins It is the thin strip of metal protruding approximately at right angle to the surface of the slab. It is caused by molten steel having run into the open cracks in the mold. Causes • Gap between the copper plate in mold • High casting superheat • Loosened/untight bolt during mold assembly • Prolong use of mold • Inadequate mold vibration Remedies • Flame cutting. • Roll edging. • Pressing
  • 52. Concavity Causes It is the distortion of the slab which appears as a concave surface. This defect is usually attributed due to improper spray cooling pattern. • Improper secondary cooling • High casting superheat • Improper mold shape • Inadequate roll pressure and roll surface profile • Cooling water quality and volume Remedies • Avoid the reasons of the defect • And flatten the slab by scarfing
  • 53. Depressions Definition Causes Depression are mainly 2 types. Longitudinal depression is a channel shaped depression on the face of the slab running in the direction of the axis. Where as transverse depression is a localized depression on the slab surface normal to the axis of the slab. • Uneven lubrication • Very rapid cooling • Low casting superheat • Mold level fluctuation Remedy Longitudinal depression Transverse depression • Avoid the reasons of the defect