defects on & in slabs
TRANSCRIPT
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DEFECTSDEFECTSON & IN SLABSON & IN SLABS
SSAB
Oh my GOD!Ugly Cracks!!
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ContentContent
Classification of defects
Type
Size
Factors influating the
cracking susceptibility.
Material properties
External factors
Short solidification theory
Calculation of shell
thicknessCrack types & causes
Actions to avoid cracking
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FactorsFactors influatinginfluating
the crackingthe crackingsusceptibilitysusceptibility..
Material propertiesSteel analysis
Temperature properties
External factors causing
tension in the strand
Mecanical
Thermal
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InfluenceInfluence ofof
analysesanalysesLow carbon, low alloy steel
has low susceptibil ity for
cracking.
Steel with carbon ~0,08-0,14%
is very susceptible to surface
cracks
High carbon, high alloy steel
has high susceptibility tointernal defects
S, P, V, Nb normally increasesthe crack susceptibility.
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PropertiesProperties atat
differentdifferent
temperaturestemperatures The ductility variates during the
solidification and cooling period.
Ts
600 900 1200 1500
Ductility
IIII II
(Fe,Mn)S-OCu,Sn,SbAlN,BN,Nb(C,N)
liquid
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SolidificationSolidification theorytheory
Internal defectsis created when
the two-phase
region is
exposed totensions
exceeding the
strength of the
material.
TS
TY
TY
Q
QTL
TY
TL
Q
TL
dT
dT
dT
dx
dx
dx
dq=-k*dTdt dx
Q=
solid
liquid
Soldification interval
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Shell growth:1Shell growth:1
It is possible to calculate theapproximate shell thicknesswith the following formula:
s=Cs* l/Vc
s=shell thickness [mm]
Cs=solidification constant (~25-27)
l =distance from meniscus [m]
Vc=casting speed [m/min]
This relatively simple formula
gives you a rough idea of theshell thickness. You must try tofind out the Cs for differentconditions.
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If you want to know where in
the machine a defect is
created you can rearrange theformula as below :
l=Vc*(s/Cs)2
The length is not exact, but it
will give you a hint where to
search for problems in yourcaster
ShellShell growthgrowth :2:2
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ExternalExternal faktorsfaktors
MECHANICAL
Taper errors
Bad mould lubricationBadly aligned rolls/segments
Skidding rolls
Strand unbending
THERMAL
Uneven cooling
Fast reheating
To soft cooling
To hard cooling
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InternalInternal
defectsdefects
1. Longitudinal narrow facecracks
2. Longitudinal cracks
3. Corner cracks4. Transversal cracks
5. Diagonal cracks
1
25
3
4
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LongitudinalLongitudinal
narrownarrow faceface crackscracksCauses:
Bulging of broad side
Pressing of narrow faceOvercooling and reheating of
the narrow face
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LongitudinalLongitudinal
crackscracksCauses :
Bulging of broad sides down in
the machineBearing- or roll damage,
misaligned rolls and/or
segments.
Insufficient cooling
Can extend to slab surface
and cause a breakout
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CornerCorner crackscracks
Causes :
Narrow face bulging
Insufficient taper
Insufficient cooling
Reheating
Can extend to slab surface and
cause a breakout
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CornerCorner crackscracks
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TransversalTransversal crackscracks
Causes:
Bulging
Deprsession of shellRoll alignment
Unbending of the strand
To high pressure from drivenrolls
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TransversalTransversal crackscracks
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DiagonalDiagonal crackscracks
Triple-point cracks
Causes:
Sensitive analyses
C-content >0,15 %
Low Mn/S ratio
Bulging
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SurfaceSurface crackscracks
1. Transversal cracks
2. Transversal corner cracks
3. Longitudional cracks
4. Short Longitudional cracks5. Longitudional corner cracks
6. Star/net cracks
3
4
5
6
1
2
5
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TransversalTransversal crackscracks
Causes:
Large temperature gradients in
the cooling zone.
Straightening at low-ductile
temp.
Misaligned/damaged/skiddingrolls can be the cause.
The cracks are normally
situated in oscillation
marks
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Transversal cornerTransversal corner
crackscracks
Causes:
Large temperature gradients in
the cooling zone.
Straightening at low-ductile
temp.
The cracks are normallysituated in oscillation
marks
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LongitudionalLongitudional crackscracks
Long, deep mid face
cracks
Causes:
Uneven shell growth due to
bad mould powder function.
Tensions generated in themould ie. bad lubrication,
scratches, insufficient taper.
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ShortShort LongitudionalLongitudional
crackscracks
Causes:
High casting temperature =>
thin shell.
Uneven shell growth.
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Longitudinal cornerLongitudinal corner
crackscracks
Causes:
Uneven corner cooling.
Bulging below the mould.
due to insufficient narrow face
cooling and/or taper
This type of crack cansome time cause severe
breakouts.
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Star/Star/netnet crackscracks
Cause:
Cupper pick-up from the mould
due to bad powder functioncauses hot shortness.
This is cupper entrained in the
grain boudaries that cause low
ductility =>cracks.
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CountermeasuresCountermeasures
Avoid tension in problemareas.
Temperature & analysis
Good steelmaking practice=> less casting problems=>
better quality.Continous supervision of
the mould area.
Always keep the casters in
top condition.Mould conditions
Segment/roll alignment
Secondary cooling