Defects on & in Slabs

8/13/2019 Defects on & in Slabs

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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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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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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

Defects on & in Slabs

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