This document describes various defects that can occur in steel billets during the continuous casting process. Section I defines shape defects such as rhomboidity, bulging, concavity, and transverse depression. Section II covers internal defects like diagonal cracking, intercolumnar cracks, and central porosity/pipe. Section III outlines surface defects including bleed outs, reciprocation marks, and scoring. For each defect, the document provides details on causes and recommended actions to check things like mould alignment, secondary cooling, casting speeds, and lubrication.

1. I.
1.1
1.2
1.3
1.4
1.5
1.6
II.
2.1
2.2
2.3
2.4
III
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
AL-TUWAIRQI STEEL PRODUCTS COMPANY
Quality Control/Quality Assurance Department
BILLET DEFECTS
Slag (Entrapped Scum)
Longitudinal Corner Depression
Transverse Corner Cracks
INTERNAL DEFECTS
Diagonal Cracking
Intercolumnar (Feathers halfway) Cracks
Central Pipe, Central Porosity
Scored Mould (ridges on billet)
Mould Meniscus Chrome Missing
Teeming Arrest
Longitudinal Corner Cracks
Skin Drag
Aluminum Entrapment
Surface Pinholes
Subsurface Blowholes
SURFACE DEFECTS
Reciprocation/Oscillation Marks
Scoring
Hot Shortness
Bleed
SHAPE DEFECTS
Rhomboidity
Bulging
Concavity
Transverse Depression (Low Carbon Steel Type)
Twist
Worn Withdrawal/Straightener Roll Marking

2. I.
1.1
(a.)
(b.)
(i)
(ii)
(iii)
D 1
D 2
Rhomboidity is the distortion from square of the billet to give differing diagonal lengths
SHAPE DEFECTS
Rhomboidity
General
Causes
Uneven mould cooling due to lack of mould taper (Flared mould or negative taper)
Uneven secondary cooling due to either misaligned or blocked sprays
═ %Rhomboidity
Action
Check mould taper
Check Secondary Cooling alignment
Check MS section of mould.
Measurement of Rhomboidity
ΔD ═
D 1- D 2
D 2
x 100
Where: is the larger diagonal length
is the smaller diagonal length

3. 1
(a.)
(b.)
(i)
(ii)
(iii)
Check secondary Cooling alignment and for blocked spray nozzles
Bulging is the distortion of one or more surfaces to produce a surface which is convex, rather
than flat.
Is due to lack of support of a thin shell against the effect of ferrostatic pressure.
This may be cause by:
Casting with excessive superheat and or fast, resulting in thinner than usual shell.
Blocked secondary cooling nozzles resulting in thinner than usual shell.
Action
Bulging
General
Causes
Check casting speed, temperature, mould water flow.
Check MS section of mould.

4. 1.3
spray nozzles.
(i)
Almost invariably has been found to be due to incorrect alignment of secondary cooling or blocked
Action
Check secondary Cooling alignment and for blocked or missing spray nozzles
Concavity
General
Concavity is the distortion of one or more faces (generally one) to produce concave rather than
flat surfaces.
Causes
Concavity and bulging may be associated on the same sample.

5. 1.4
(ref.diagram).
i.e
(i)
Transverse Depressions
General
Check and control casting speeds to SOP range.
Localized depressions across the face of the billet, often being present on all four faces in a
'zone of defect' around the billet.
Causes
Casting too fast for combination of steel type / mould taper.
Billet shrinkage insufficent for mould taper resulting in excessive drag. In extreme instances the billet
can be seem to jump upwards in the 'loop' area (pre-withdrawal drives). This results in the billet
actually 'necking' as in tensile testing with the resulting development of cracks. If the sticking is
severe enough, it will result in breakouts.
The types of steel most prone to this are the very low carbon grades.
ex. 701, 702 (1005 type steels)
Action

6. 1.5
(i) Check for missing / blocked nozzles in the spray apron.
(ii) Check for leaking hoses, pipes or misdirected spray cooling jest in the withdrawal
drive / strainer unit area .
Action
This defect is normally associated with uneven cooling in the spray zone and particularly in the withdrawal
drive/straightener unit area.
General
Twist is when the billet appears to have been twisted around its longitudinal centreline axis.
Causes
Twist

7. 1.6
(i)
Check for alignment and indications on the billet surface.
Worn withdrawal/Straightener Roll Marking
General
These are markings that appear longitudinally all thorughout the billet length most of the times
in regular frequency and pattern.
Causes
This defect is normall associated with worn-out/damaged or unaligned roller surfaces usually on the
straightener area.
Action
Check and change rollers periodically or monitor useful life of roller unit.
(ii)

8. II.
2.1
(a.)
(i)
(ii)
This is generally found associated with rhomboidity, in particular rhomboidity caused by
secondary cooling where there has been twisting of a reasonably thick shell.
Rhomboidity associated with mould defect (lack of taper) does not tend to exhibit this defect to
the same extent.
Often results in a breakout htrough the crack.
Uneven secondary cooling of the billet which gives rise to stress which results in cracks
opening on the corner planes of weakness.
Causes
Check secondary cooling alignment
If secondary cooling appears satisfactopry, check for lack of mould taper
INTERNAL DEFECTS
Diagonal Cracking
General
Action

9. 2.2
(a)
(b)
(i)
(ii)
General
Causes
These form approximately 1/3 to 2/3 of the way between the outside faces and the centre. They
are generally 'hairline' and cannot be seen without macroetching. The location of the cracks
(closer to the outside or centre) is indicative of the relative location of the problem (high or low)
in the cooling zone.
Check casting practice against S.O.P.
Due to thermal stress, the result of uneven secondary cooling
Due to reheating of the billet after excessive cooling (i.e low speed, high water flow).
Action
Check secondary cooling alignmnet and for blocked or missing spray nozzles.
INTERCOLUMNAR FEATHER/HALFWAY CRACKS

10. 2.3
Check casting practice again, against S.O.P., though the 'defect' does not tend to produce rod defecrts in
itself, it may well indicate other potential problems such as segregation on high carbon grades.
CENTRAL PIPE, CENTRAL POROSITY
General
Causes
Central pipe being when there is a defiite single hole in the center of the billet, the hole being of
intermittent nature and extending logitudinally down the billet.
Central porosity is when the center of the billet exhibits a general unsoundness (can sometime
only be seen after macroetching).
(a)
The smaller the section size, the easier it is for the inward growing shell to bridge across the molten
core from surface irregularities and hence create isolated pockets of molten metal which on
solidification leave voids. (solidification shrinkage of steel is approx 7%).
If the temperature (superheat) is high then the molten core is long and narrow and eaasier to bridge
across.
Associated with a number of casting factors such as section size, casting speed and temperature.
Action
The casting speed effect is that if casting fast, the molten core is long and narrow and easier to
bridge across.
(b)
(c)

11. 2.4
(a)
Action
Often the holes can be seen on the billet ends during casting. Providing it is compatible with
specification chemistry, increase the aluminum wire feed rate to the strand.
( i )
If the problem is due to a wire feeder being out of service, plug-off the strand.
SUBSURFACE BLOWHOLES
General
These voids are intercolumnar grain, elongated 5-20mm length in the direction of grain growth.
If they reach the surface or are sufficiently close to the surface that upon reheating of the billet
they will be exposed to air and react to form oxide scale, then they will cause a rod defect.
( ii )
Often the steel may be seen to be rimming slightly in the mould. If this is the case, check the
aluminum feed rate and increase if necessary.
General case
The result of insufficient deoxidation of the steel, the excess oxygen being evolved on solidification (
i.e. the steeel rimming.)
C + O ⇄ CO
Formation of an oxide layer prevents these holes welduing up during rolling.
Cause

12. (b)
Hydrogen arises from two sources, the use of organic binders in cold tundish lining tiles and from the
natural gas use as the shrouding gas on the tuyeres of the converters. It does not normally cause
blowholes by itself but in conjunction with [O] and [N], the higher [H] at start of cast (on a new
tundish) from tile pick up can result in from end blowholes on the first billets from each strand.
Special case
In addition to [O], [N] and [H] can alos casue rimming in the mould and subsequent subsurface
blowholes.
( ii )
The solubility of [N] in pure Fe is 0.043 by weight i.e. 430 ppm. This can result in substantial nitrogen
gain on low carbon steels depending upon steelmaking and LTS practices. In practice for [N], values
of greater than approximately 140ppm [N], are sufficient to cause blowholes. The actual level at
which subsurface blowholes due to [N] will occur will also be dependent upon the residual [O] values
which are dependent upon the Mn and Si levels.
Cause

13. III
3.1
(a.)
(i)
(d.)
May be due to lack of mould taper resulting in a thin shell exiting the mould. Lack of billet/mould
contact will result in a thinner shell due to reduced heat transfer.
If persistent bleeds/breakouts are associated with a particular mould over two consecutive
heats, change the mould.
(ii)
Action
Check secondary cooling for blocked nozzles.
(c.)
Often associated with "dirty" steel (stream condition/stell chemicstry) where a slag inclusion has
fallen out.
SURFACE DEFECTS
BLEED
General
A bleed occurs when molten steel core penetrate through the steel skin. This can occur either
while within the mould or below the mould. If the leak is chilled off by either the mould or the
secondary cooling sprays it is termed a bleed. If casting of the strand has to terminate due to
loss of the molten core from the mould, then it is termed a breakout.
Excessive superheat results in thinner shell hence greater risk.
Excessive casting speed, due to oxygen lancing of a nozzle open and burning out of the bore, will
give thinner shell.
(b.)
Causes

14. 3.2
(a)
(b)
( i ) _
_
_
oil slits not blocked
supply lines have no holes
pumps working
Generally the problem is related to blocked oil slits.
Check mould lubrication
RECIPROCATION/OSCILLATION MARKS
General
These are transverse, parallel marks, the distance between the marks being related to the
amount that the billet has been withdrawn during one oscillation cycle.
Some degree of marking is unavoidable but excessive ridging can cause lapping defects on
rolling.
Action
Causes (of heavy marking)
Incorrect oscillation stroke/cycle rate, for the casting speed.
Lack of mould lubrication.

15. 3.3
Causes
(a)
(b) A foreign object, often a bit of a bleed or breakout becoming lodged in the secondary cooling
system or withdrawal/straightener area and scoring the billet.
( C) Tracking to one side of the strand guide and rubbing against a strand deflector guide plate.
If detected during cast, check the billet at stages down through the machine to determine general
location of the problem.
NB:
General
This a groove gouged longitudinally along the billet face.
Narrow deep marks are worse than wide shallow gouges as they are more likeky to lap over
during rolling, resulting in a seam in the rod.
If unable to detremine location of if cast has finished, check strand top to bottom.
Do not check in danger areas such as the secondary cooling area during cast.
Action
SCORING
Seized roller with associated scale build up.

16. 3.4 HOT SHORTNESS
General
This is found when the billet is deformed under tensile conditions at elevated temperatures (i.e.
hot shearing). It is not evident if the billet has been gas cut to length.
The defect shows as a maze of fine transverse cracks on the tensile face of the shear cut.
Causes
The defect is caused by grain boundary precipitation of copper, the problem being accentuated by a 'high'
tin level. This copper, copper/tin precipitate has a low melting point and hence low hot tensile strength.
Action
Check scrap source for copper or brass. If high levels of both tin and copper are present, often from
soldered products scuch as vehicles radiators.

17. 3.5
The defect may be present on one or all faces.
(a)
(b)
(c)
(d)
(i)
(ii)
(e)
Check oil slits for skull build up.
SKIN DRAG
General
Causes
Insufficient mould lubrication (rapeseed oil flow).
Action
This is when the billet has been sticking in the mould with the result being that the oscillation
marks are no longer reasonably neat parallel lines. They will be dragged out of parallel,
generally in the center of the billet face.
Broken, perished or folded oil supply line.
Rapeseed oil pump u/s.
Blocked oil delivery slits in the mould.(this being the most common cause.)
"Crazed" chrome plate in the miniscus region due to either high mould life or due to the mould being
overheated at sometime (either lack of water or deposit of CaPO 4 ).
Check oil flow rate, lines pump.

18. 3.6
Silver/yellow silver entrapped inclusions on the surface of the billet are aluminum from the
mould wire feed. If present they are often at regular intervals along the billet face.
Causes
ALUMINUM ENTRAPMENT
The wire should be fed into the tundish stream between just above the meniscus to approximately 25mm
above.
If fed into the molten metal in the mould, particulary is close to one wall, fluctuations in mould metal level
can cut off pieces which float on top of the steel and chill to the meniscus.
Action
Explain the correct location to feed the wire into the mould.
Feed Location
Incorrect feeding position of the wire into the mould.
General

19. 3.7
(a)
(i )
(ii ) Check that oil delivery slits are clear and that oil flow is as per S.O.P.
SURFACE PINHOLES
General
These are found on the billet surface and differ from subsurface blowholes in that they tend to
be 1-3mm in depth where as subsurface blowholes which may occasionally breach the surface
range from 5-20mm in depth.
Causes
Water contamination of the rapeseed oil (spec. 0.03% max.) in practice 0.04% has not given
problems.
Action
(c)
Too high a gas content in steel O 2 + N 2 + H 2 , this can form surface pinholes (in addition to
subsurface holes).
(d)
Check Aluminum feeders are feeding S.O.P. wire feed rates.
They may be conical shaped (point down) or wide shallow craters, affecting all wire grades.
Insufficient mould deoxidation (too low a rate of aluminum wire feed).
Grossly unbalanced lubricating oil flow inot the mould. That is most or all of the oil be delivered to
one face due to blocked delivery slits.
(b)

20. 3.8
NB:
Action
Due to the solidification of the molten core, the defect will extend fro approximately
1000mm down strand from the actual seam on the outside of the billet.
Any billet with a teeming arrest must be either cut back to a short or scrapped.
TEEMING ARREST
General
A teeming arrest occurs when there is a temporary stoppage in the casting of a billet. That is,
the steel is deliverted away from the mould for a short period (laundering).
This results in the solidification of the molten core that was present. Upon restarting, the
moplten steel welds imperfectly to the solid steel present creating conical zone of weakness.
This may tear apart during hot rolling or will create a zone of weakness in the rolled produce.

21. 3.9
(a)
(b)
(i)
(ii)
Check mould.
Check secondary cooling.
This is a depressed groove on the surface of the billet, adjacent to the corner, extending down
the length of the billet.
The defect is often associated with rhombhoidity and longitudinal corner cracks.
Flared mould.
Action
Uneven secondary cooling.
LONGITUDINAL CORNER DEPRESSION
General
Cause

22. 3.10
(a)
(b)
(c)
LONGITUDINAL CORNER CRACKS
Some steels have on past experience proved more prone to cracking, those of approximately
0.05%C and 0.20 wt %C.
Causes
General
Check mould taper and secondary cooling equipment.
Lack of mould taper.
Distortion of mould meniscus area.
Action
Secondary cooling alignment.
These are cracks that occur on the round corner radius of the billet and extend down the length
of the billet. The crack may be continuous or intermittent.

23. 3.11
(a)
(i)
(ii)
(iii)
Action
(b)
If deep and long, are often due to misalignment of the secondary cooling give cold
corners which cracks under the tensile stress of bending straightening.
(c)
Casting slow and cold due to a tundish nozzle problem can also result in cracks because
of steel temperature at bending/straightening.
Check casting speed/temperature of affected stand.
TRANSVERSE CORNER CRACKS
General
These occur on the corner of the billet. They may or may not extend across the full face of the
billet towards the near corners.
They often occur on reciprocation marks (these being places of slight weakness due to welding
folding of the shell during the oscillation cycling).
Check secondary cooling alignment.
Causes
If very shallow, caused by sticking of the billet in the mould.
Check mould lubrication, especially for blocked oil delivery slits.

24. 3.12
Action
Depending upon the severity of the ridge and the grade of steel being cast the mould should be
removed from service.
SCORED MOULD (RIDGES ON BILLET)
General
Cause
Slight ridges on the face of the billet (often 1mm x 1mm) and generally in the corner area are
the result of scoring in the mould meniscus area .The ridge may be continuous or intermittent,
depending upon the relationship of the score to the meniscus area of the mould which is
oscillation up and down.
If a mould is scored in the meniscus area, the solidfying shell form this shape.

25. 3.13
A deposit on the outside of the mould will inhibit the heat transfer allowing the mould to heat up
in the insulated area. Because of the large differences in the expansion rates of copper and
chrome, the chrome tends to spall off.
MOULD MENISCUS CHROME MISSING
General
This results in a characteristic form of bleed which is due to tearing of the thin shell in the
meniscus area. In general, the defect is samall, the shell tearing only over the defect, rather
than across the full faces of the meniscus.
Cause
Chrome missing from a spot in the mould meniscus area.
In general, the defect appears almost continuous, occurring on virtually every oscillation band
(dependent upon how steady the casting level is maintained).
Action
Remove mould from service. Check as soon as possible for calcium or phosphate deposit on
the outside of the meniscus area. If present, all moulds should be removed as soon as practical
for cleaning to remove the build up.

26. 3.14
(b)
Action
Causes
Origin is from either deoxidation or reoxidation products.
Deoxidation - carried over from ladle due to chemistry/physical nature of material
Reoxidation - from eihtr reoxidation of alloying in elements in the tundish or mould due to
contact with air (landle and tundish streams)
If not removed from mould (fished out) while floating on the surface, it may/will become
entrapped on the solidyfying meniscus and travel down the mould, creating a depression in the
steel which solidifies around the slag.
(c)
If the problem is due to a poor stream condition on a single strand, then plug-off the strand if
casting conditions (temperature) permit.
(a)
Poor tundish nozzle stream condition will greatly increase the amount of slag generated
and hence the likelihood of a slag problem.
Insufficient fishing/skimming by operator.
Some grades are inherently more susceptible, particularly those with low Mn:Si ratios.
(i.e,. 2.5:1).
General
SLAG (ENTRAPPED SCUM)