Pin holes as a surface defect, and blow holes as an inner defect, occur in billet / bloom / beam blank casting, particularly for Si-Mn killed steel cast with metering nozzle and oil lubrication. If they are present in abundance or have a large size, they may originate defects in the rolled products. Their characteristics, factors behind their formation, as well as usual processing conditions that promote their occurrence are discussed. Finally, the evolution of these defects during reheating and rolling is analyzed, with industrial examples

1. August 20, 2015
Billet Defects: Pin Hole and Blow Hole
Formation, Prevention and Evolution
Jorge Madias, Alberto Moreno, Cristian Genzano
metallon, San Nicolas, Argentina

2. Content
Introduction
Pin Holes
Characterization
Formation
Evolution during rolling
Countermeasures
Blow Holes
Characterization
Formation
Evolution during rolling
Countermeasures
Conclusions

3. Introduction
Consulting & Training for the Steel Industry
Technical assistance
Open, in company and self-learning
courses
Library services
Met Lab services
Technical texts for trade journals

4. Introduction
Content based on
Experience of the three authors while working for
steel companies to troubleshoot surface and
internal defects in billets, blooms and slabs
Material prepared for short course on billet and
bloom casting held once a year in Argentina and
several times in different steel companies in Latin
America and the Middle East

5. Introduction - Pin holes & Blow holes
Pin holes as a surface defect, and blow holes as an inner defect,
occur in billet / bloom / beam blank casting, particularly for Si-Mn
killed steel cast with metering nozzle and oil lubrication
If they are present in abundance or have a large size, defects
may occur in the rolled products
Their characteristics, factors behind their formation, as well as
usual processing conditions that promote their occurrence are
discussed
Finally, the evolution of these defects during reheating and
rolling is analyzed, with industrial examples

6. Pin Holes - Characterization
Pin holes are detected with naked eye on the as cast surface,
but sandblasting or shot blasting let us see much more

7. Pin Holes - Characterization
Distribution of the pin-holes on billet surface
depends on the phenomena that originated its
formation
Transverse belts
Longitudinal occurrence
Other patterns

8. Pin Holes - Characterization
Transverse cutting

9. Pin Holes - Formation
Associated with
Gas development by pyrolysis of lubricating oil
Presence of oxygen dissolved in the steel.
Oil rate
Friction surface defects are not very sensitive to
oil rate (within a certain range)
20-30 ml/min typical

10. Pin Holes - Formation
Oil rate and friction
S. Chandra,
PhD thesis

11. Pin Holes - Formation
Often, it is not the case of excessive oil rate at a
given time, but an inhomogeneous distribution of the
oil around the billet section
A divided box located inside the mold during inter-
sequence time let us have an idea of oil distribution
This can be related with pin-holes distribution in the
billets

12. Pin Holes - Formation
Oil distribution and pin hole formation

13. Pin Holes - Formation
Factor in oil distribution: lubrication slot gap
Usually recommended to be 1 mm max
Spitting may partially close the gap
Before improvements After improvements End of campaign

14. Pin Holes - Formation
Influence of oxygen and other gases dissolved in the
steel
Low carbon steel billets more sensible
First billets of a casting sequence, submitted to
reoxidation and hydrogen pick-up during tundish
filling.
Moisture in the oil or picked –up in the oil circuit has
long being recognized as a factor
Sudden variation of meniscus level is another factor

15. Pin Holes – Evolution during rolling
Deeper pin holes survive reheating
Billet preheated but not rolled

16. Pin Holes – Evolution during rolling
Pin hole are filled with scale during reheating
Billet preheated but not rolled

17. Pin Holes – Evolution during rolling
Surviving pin holes elongate during rolling
They may disappear if reduction is large (wire
rod)

18. Pin Holes – Evolution during rolling
In metallographic observation, rolled pin holes often
are thicker in the inner part than outside

19. Pin Holes – Countermeasures
In relation with oil pyrolisis
Automation of the lubrication system
Change to vegetable oils modified with esters
Proper maintenance of the lubrication circuit
Favorable effect of electromagnetic stirring

20. Pin Holes – Countermeasures
In relation with dissolved gases
Addition of deoxidizers (Titanium, Al injection)
At a cost
 Side effects, as slag patches and breakouts, for Al injection
Avoiding reoxidation by use of ladle shroud and gaseous tundish stream
protection
At a cost, but beneficial for quality, safety and working environment
Cases
Aceros Arequipa, 2009:
Improvements in pin-holing by decreasing tundish height (shorter tundish to
mold stream), changing the type of ladle shroud and by automation of the
lubrication system
Godo Steel, 2013:
Using ladle shroud and argon protection for the tundish stream, eliminated
aluminum wire injection to avoid slag patches, increasing at the same time
aluminum addition at tapping and using CaSi cored wire injection

21. Blow Holes – Characterization
Typically, blowholes are
accommodated relatively closed to
the billet shell, with a
perpendicular direction
If very serious, they can be seen
even in the oxy cut
Distribution (location in the
sequence; strands affected; billet /
bloom length affected, etc.), varies
from case to case
depends on the root cause for
the excess of gas in the steel

22. Blow Holes – Characterization
Blow holes start in
interdendritic regions
They end at a given
distance from the
billet/bloom/beam
blank skin

23. Blow Holes – Characterization
Blow hole details (Oberhofer etching)
Billet skin
Blow hole start
Blow hole end
(convex)
Segregation
Blow hole start
(concave)

24. Blow Holes – Characterization
Blow holes and segregation

25. Blow Holes – Characterization
Gas analysis in blow holes
Dragon Steel Corporation, 2010
90.7% H2
8.9% CH4
0.4% CO2
0.2% CO

26. Blow Holes – Formation
Generation attributed to an
excess of dissolved gases
(oxygen, nitrogen and / or
hydrogen, enough to produce
a bubble
This process has been
modeled from early billet
casting times, giving a base to
understanding of the
phenomena

27. Blow Holes – Evolution during rolling
These defects, if not in contact with the surface, will not develop
scale during reheating, as opposite to pin holes
So, they may weld partially or totally during rolling
Segregation is associated with them and may cause trouble in
certain applications
This segregation is often seen in the transverse metallographic
cut of rolled products as ghost lines
They could also arise from other segregation-related defects,
like midway and off-corner cracks
If the occurrence of blowholes in certain operating situations is
endemic, an improvement plan should be designed and carried
out

28. Blow Holes – Countermeasures
Minimizing the occurrence of the blowholes is a question of
constant and uniform operating practices avoiding the risk of high
oxygen, nitrogen or hydrogen in the steel
dry lime (or partial use of sintered lime) in the ladle
low furnace slag carry-over
an even deoxidation practice that avoids dispatching un-
deoxidized or over-deoxidized ladles to the caster
avoiding long treatment times
switching to argon when this occurs
tundish and ladle without remaining moisture

29. Blow Holes – Countermeasures
Typical operating situations where blowholes may occur
High Oxygen
Heat sent to the caster when deoxidation was not still finished, due for
instance to coordination reasons or difficult deoxidation related to slag
carry over
High Oxygen and Nitrogen
Start of a sequence. Here the oxygen and nitrogen picked-up by contact
with air during tundish filling may be enhanced by moisture remaining in
the tundish lining
High Hydrogen
New ladle with some moisture remaining in the lining
High Nitrogen
Ladle with long treatment time, when just nitrogen is used as stirring gas

30. Blow Holes – Countermeasures
Cases
Beam blank caster (Dragon Steel Corporation,
2011)
Main contributor to blowhole formation was
hydrogen from the sealing material for tundish
lining
Billet caster (Tata Steel Thailand, 2012)
Focus was on improving deoxidation and
avoiding reoxidation
Billet caster (CSN, 2015)

31. Conclusions
Pin holes and blow holes are typical defects that occur in Si-Mn killed steel
cast with metering nozzle and oil lubrication
Although in both cases the root cause for their formation is gas evolution,
the mechanisms are somewhat different
In the formation of pin holes, the gas generated by the pyrolysis of the
lubricating oil seems to be the most important factor. Nevertheless, some
synergy with gases dissolved in the steel might be possible
Instead, blow holes for theses steels are originated in gases dissolved in
the steel that segregate to interdendritic space and if their partial pressure is
enough, form bubbles. For the bubbles to develop, besides a high content in
dissolved gas, segregation of gases to interdendritic spacing is required, a
condition that is not met at the surface, but to some depth inside the billet
Although these billet defects not always give place to defects in the rolled
products, their occurrence must be closely followed and avoided

32. Thank You!
Jorge Madias, Alberto Moreno, Cristian Genzano
metallon, San Nicolas, Argentina
www.metallon.com.ar