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DEFECTSDEFECTSON & IN SLABSON & IN SLABS

SSAB

Oh my GOD!Ugly Cracks!!

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ContentContentClassification of defects

TypeSize

Factors influating the cracking susceptibility.

Material propertiesExternal factors

Short solidification theoryCalculation of shellthicknessCrack types & causesActions to avoid cracking

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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)

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FactorsFactors influatinginfluatingthe cracking the cracking

susceptibilitysusceptibility..Material properties

Steel analysisTemperature properties

External factors causingtension in the strand

MecanicalThermal

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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.

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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

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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

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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.

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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

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ExternalExternal faktorsfaktors

MECHANICALTaper errorsBad mould lubricationBadly aligned rolls/segmentsSkidding rollsStrand unbending

THERMALUneven coolingFast reheatingTo soft coolingTo hard cooling

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InternalInternaldefectsdefects

1. Longitudinal narrow face cracks

2. Longitudinal cracks3. Corner cracks4. Transversal cracks5. Diagonal cracks

1

25

3

4

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Longitudinal Longitudinal narrownarrow face face crackscracks

Causes:Bulging of broad sidePressing of narrow faceOvercooling and reheating of the narrow face

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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

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Corner Corner crackscracks

Causes :Narrow face bulging

Insufficient taperInsufficient cooling

Reheating

Can extend to slab surface and cause a breakout

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Corner Corner crackscracks

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Transversal Transversal crackscracks

Causes:BulgingDeprsession of shell

Roll alignmentUnbending of the strandTo high pressure from driven rolls

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Transversal Transversal crackscracks

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Diagonal Diagonal crackscracks

”Triple-point cracks”Causes:

Sensitive analysesC-content >0,15 %Low Mn/S ratio

Bulging

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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

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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

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Transversal corner Transversal corner crackscracks

Causes:Large temperature gradients in the cooling zone.Straightening at low-ductiletemp.

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 the mould ie. bad lubrication, scratches, insufficient taper.

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ShortShort LongitudionalLongitudionalcrackscracks

Causes:High casting temperature => thin shell.Uneven shell growth.

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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.

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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.

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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