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© Peter Kruse
DEFECTSDEFECTSON & IN SLABSON & IN SLABS
SSAB
Oh my GOD!Ugly Cracks!!
© Peter Kruse
ContentContentClassification of defects
TypeSize
Factors influating the cracking susceptibility.
Material propertiesExternal factors
Short solidification theoryCalculation of shellthicknessCrack types & causesActions to avoid cracking
© Peter Kruse
ClassificationClassificationDefect types
Shape defects kod 30-39Surface defects kod 40-49Other actions kod 50-59Internal defects kod 60-69
SizeMinor, relatively harmless defects(0-3mm)Semi deep defects(4-11mm; treatment nessesary)Deep defects(12-20mm; Heavy treatment)Very deep defects(> 20mm: could be scrap)
© Peter Kruse
FactorsFactors influatinginfluatingthe cracking the cracking
susceptibilitysusceptibility..Material properties
Steel analysisTemperature properties
External factors causingtension in the strand
MecanicalThermal
© Peter Kruse
InfluenceInfluence of of analysesanalyses
Low carbon, low alloy steelhas low susceptibility for cracking.
Steel with carbon ~0,08-0,14% is very susceptible to surfacecracks
High carbon, high alloy steelhas high susceptibility to internal defects
S, P, V, Nb normally increasesthe crack susceptibility.
© Peter Kruse
PropertiesProperties at at different different
temperaturestemperaturesThe ductility variates during the solidification and cooling period.
Ts
600 900 1200 1500
Duc
tility
IIII II
γ γγ
(Fe,Mn)S-OCu,Sn,SbAlN,BN,Nb(C,N)
γ liquid
© Peter Kruse
SolidificationSolidification theorytheory
Internal defectsis created whenthe two-phaseregion is exposed to tensions exceeding the strength of the material.
TS
TY
TY
Q
QTL
TY
TL
QTL
dT
dT
dT
dx
dx
dx
dq=-k*dTdt dx
Q=
solid
liquid
Soldification interval
© Peter Kruse
Shell growth:1Shell growth:1
It is possible to calculate the approximate 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 formulagives you a rough idea of the shell thickness. You must try to find out the Cs for different conditions.
© Peter Kruse
If you want to know where in the machine a defect is created you can rearrange the formula 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 your caster
Shell Shell growthgrowth :2:2
© Peter Kruse
ExternalExternal faktorsfaktors
MECHANICALTaper errorsBad mould lubricationBadly aligned rolls/segmentsSkidding rollsStrand unbending
THERMALUneven coolingFast reheatingTo soft coolingTo hard cooling
© Peter Kruse
InternalInternaldefectsdefects
1. Longitudinal narrow face cracks
2. Longitudinal cracks3. Corner cracks4. Transversal cracks5. Diagonal cracks
1
25
3
4
© Peter Kruse
Longitudinal Longitudinal narrownarrow face face crackscracks
Causes:Bulging of broad sidePressing of narrow faceOvercooling and reheating of the narrow face
© Peter Kruse
Longitudinal Longitudinal crackscracks
Causes :Bulging of broad sides down in the machine
Bearing- or roll damage, misaligned rolls and/or segments.Insufficient cooling
Can extend to slab surfaceand cause a breakout
© Peter Kruse
Corner Corner crackscracks
Causes :Narrow face bulging
Insufficient taperInsufficient cooling
Reheating
Can extend to slab surface and cause a breakout
© Peter Kruse
Corner Corner crackscracks
© Peter Kruse
Transversal Transversal crackscracks
Causes:BulgingDeprsession of shell
Roll alignmentUnbending of the strandTo high pressure from driven rolls
© Peter Kruse
Transversal Transversal crackscracks
© Peter Kruse
Diagonal Diagonal crackscracks
”Triple-point cracks”Causes:
Sensitive analysesC-content >0,15 %Low Mn/S ratio
Bulging
© Peter Kruse
SurfaceSurface crackscracks
1. Transversal cracks2. Transversal corner cracks3. Longitudional cracks4. Short Longitudional cracks5. Longitudional corner cracks6. Star/net cracks
3
4
5
6
1
2
5
© Peter Kruse
Transversal Transversal crackscracks
Causes:Large temperature gradients in the cooling zone.Straightening at low-ductiletemp.Misaligned/damaged/skidding rolls can be the cause.
The cracks are normallysituated in oscillation marks
© Peter Kruse
Transversal corner Transversal corner crackscracks
Causes:Large temperature gradients in the cooling zone.Straightening at low-ductiletemp.
The cracks are normallysituated in oscillation marks
© Peter Kruse
LongitudionalLongitudional crackscracks
Long, deep ”mid face cracks”Causes:
Uneven shell growth due to bad mould powder function.Tensions generated in the mould ie. bad lubrication, scratches, insufficient taper.
© Peter Kruse
ShortShort LongitudionalLongitudionalcrackscracks
Causes:High casting temperature => thin shell.Uneven shell growth.
© Peter Kruse
Longitudinal corner Longitudinal corner crackscracks
Causes:Uneven corner cooling.Bulging below the mould.due to insufficient narrow face
cooling and/or taperThis type of crack cansome time cause severebreakouts.
© Peter Kruse
Star/Star/netnet crackscracks
Cause:Cupper pick-up from the moulddue to bad powder functioncauses ”hot shortness”.This is cupper entrained in the grain boudaries that cause lowductility =>cracks.
© Peter Kruse
CountermeasuresCountermeasures
Avoid tension in problem areas.
Temperature & analysisGood steelmaking practice=> less casting problems =>better quality.Continous supervision of the mould area.Always keep the casters in top condition.
Mould conditionsSegment/roll alignmentSecondary cooling
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