The document discusses off-corner cracks as internal defects in steel billets and blooms, exploring their formation mechanisms, prevention strategies, and evolution during rolling. Key findings indicate that these cracks are affected by factors such as mold design, metallurgical measures, and operational conditions, with a focus on high casting speeds and specific steel compositions. The research highlights the importance of troubleshooting and optimizing conditions to minimize the occurrence of these defects in the steel manufacturing process.

1. Billet Defects: Off-corner Cracks
Formation, Prevention and Evolution
Jorge Madias
metallon
Consultant

2. Content
Introduction
Characterization
Formation mechanism
Prevention
Evolution during rolling
Conclusions

3. About metallon
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foundry industry, based in San Nicolas, Argentina
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References
ArcelorMittal
Gerdau
Tenaris
Ternium

4. Introduction
Drivers behind preparation of the review
Series of review papers on billet, bloom and beam
blank defects at AISTech, since 2011
Troubleshooting work on off-corner cracks at
several Latin American steel companies

5. Introduction
 Longitudinal off-corner cracks
 Internal defects that may occur in billets, blooms, slabs, thin slabs
and beam blanks
 Solidification cracks located in some of the eight typical locations,
perpendicular to the surface
 One or more parallel cracks at a given position
 Several mm below the surface, but under certain conditions they
may open to the surface
 Often, they appear below a surface depression
 Manganese sulfides may refill them.
 Continuous casting
 In extreme case, shell breakouts
 Rolling
 Partially or complete welding
 Manganese sulfides remain
 Machining, forging or heat treating
 Defects may occur

6. Characterization
Macroetching or Baumann printing a transverse
cut of a billet / bloom
130 x 130 mm SAE 1030 steel billet cast
with metering nozzle and oil lubrication,
macroetch with HCl 50%, 70oC

7. Characterization
Comparison with a chart (plant specific)
Grado I Grado II
Grado IV
Acciaierie Venete, Padova, Italy

8. Characterization
Countings for improvement plan
Positions affected
Minimum distance to the billet surface
Crack length
Total crack length in a transverse cut

9. Characterization
Path between columnar grains, occupying
interdendritic spaces
Partially filled with manganese sulfides
For high Cr steels, like valve steels, chromium
carbides may take part in the refilling of the cracks
Macroetching of 240 x 240 blooms; Acciaierie Venete, Padova, Italy

10. Formation mechanism
 Early research by Brimacombe, Hawbolt
and Weinberg
 Sampling in three billet casters
 Design and operating conditions typical
of those times (short molds, some of
them straight, small sections, low
casting speed)
 Macroetching, Baumann prints,
metallography with several etchants,
cracking under liquid N, SEM
observation of the open cracks
 Solidification modeling
 Off-corner cracks attributed to faster
contraction in billet corners; separation of
the mold, early air gap formation
 Then, less heat transfer in the corner
zones
 Solid shell thinner than in face center
 Down the mold, and after mold exit,
when ferrostatic pressure is enough,
bulging occurs
 Tensile forces acting on the thin shell
may start the crack

11. Formation mechanism
More recent research by Drs. Park and Brian
Thomas, including the effect of the air gap in a
solidification model, came to the conclusion that
the corner is hot, not cold
The same tensile stress by bulging in the lower
mold or after exit, give place to
Internal off-corner cracks with smaller mold radius
Open corner cracks with larger mold radius
 Powder casting better than oil, given certain
conditions

12. Formation mechanism
Small and large corner radius; oil vs. powder

13. Formation mechanism
Critical casting speed to avoid off-corner cracking

14. Prevention
Mold design
Metallurgical measures
Operating conditions (mold and first cooling zone)
Summary of trobleshooting at several plants

15. Prevention
Mold design
Improvements in mold design are multipurpose,
including measures to solve rhomboidity, off-corner
cracks and other solidification defects, particularly
at high casting speeds
Mold taper moved from single to double to three /
four different tapers to parabolic taper
Then to transverse variations in taper like
CONVEX and DIAMOLD molds, and modifications
in water cooling like in INVEX and Powermold

16. Prevention
Mold design
CONVEX mold
Transverse changes in taper

17. Prevention
Mold design
DIAMOLD mold
Differentiated taper in the corners

18. Prevention
Mold design
INVEX mold
Slots in water side
Differentiated taper in the corners
Large radius without longitudinal corner cracking
150 x 150 mm billet mold 265 x 385 mm bloom mold

19. Prevention
Mold design
Powermold / Ecopower mold
Holes in the copper, replacing the gap between
water jacket and mold

20. Prevention
Metallurgical measures
Mn/S ratio (depends on sulfur content)
Mn/Scritical=1,345 x S (-0.7984)
SAIL Ranchi, India
125 x 125 mm medium
carbon steel billets for
forging, open casting

21. Prevention
Metallurgical measures
Superheat
Higher superheat: more cracking
Less shell thickness, easier bulging and more
probability of off-corner cracks formation
60 billets of HVN valve
steel, submerged cast,
Villares Metals, Sumare
plant, Brazil

22. Prevention
Metallurgical measures
Chemical analysis
Most of the studies on off-
corner cracks relates to
steels in the range of 0.15 –
0.35% C
 Partial coincidence with the
range favorable for higher
rhomboidity
0,35% C is prone to off-
corner cracks, in particular
when taper is small
Cracks reported in low and
high carbon steels, too
It seems that steels outside
the peritectic range are
more prone to off-corner
cracks

23. Prevention
Operating conditions – mold
Casting speed
In a previously shown case, there is a positive effect of
increasing casting speed on off-corner cracking
One possible reason for this is that the taper, if optimized for
high casting speed, is not enough for a lower speed, giving
place to a higher gap, bulging and off-corner cracking
Instead, a case on high carbon steel reports the opposite
Primary cooling
Application of more intense primary cooling gave better
results in two different plants

24. Prevention
Operating conditions – mold
Mold flux
The lower the mold flux viscosity (and basicity), the
higher the heat transfer
This promotes a thicker shell, minimizing bulging and off-
corner cracking

25. Prevention
Operating conditions – Mold
Mold flux
Tata Steel Thailand, high carbon steel
Heat extraction higher in strands cast with the flux of lower
viscosity
With low viscosity flux, heat extraction increased with
increasing casting speed

26. Prevention
Operating conditions – mold
M-EMS
Very short columnar zone, followed by equiaxed
solidification, should impair off-corner cracking
Proven in ArcelorMittal Monlevade

27. Prevention
Variable AM
Monlevade
, Brazuk
Villares
Metal,
Brazil
Tata Steel
Thailand
Gerdau
Charqueadas
, Brazil
Acciaierie
Venete
CCM3, Italy
AM
Piracicaba
,
Brazil
SAIL Ranchi,
India
Sideno
rBasau
ri,
Spain
Section (mm) 155 x 155 Bloom 155 x 155 240 x 240 120 x 120 125 x 125 145 x
145
170 x
170
Casting type SEN SEN SEN SEN SEN Open Open SEN
Steel types 1035 HNV3
304 L
0.6-0.75%C 0.15-0.25%C
0,04-0,07%S
0,15-0,25%C
>0,02%S
Ti-B
0.26-
0.32%
Medium C
forging
12L14
Mn/S Constant
(>30)
15-20
20-25
optimum
20-30
>25 ideal
3.2-4.8
Superheat (oC) Constant
+/- 5oC
The
lower
the
better
The lower
the better
35 typical 30-50 no
influence
48+/-
12
Casting speed
(m/min)
Constant 1.7 -2.2
1.7-2.0
optimum
1,7-2,5
Optimum
2,2-2,5
1.5-1.7
(145)
1.3-1.7
(170)
Oscillation
parameters
Constant Low
frequenc
y better
Stroke 5 mm Consta
nt
Mold
material,
Cu-Ag
Cu-Zr 780 >330
Constant Consta
nt

28. Prevention
Variable AM
Monlevad
e
Villares
Metals
Tata Steel
Thailand
Gerdau
Charqueadas
Acciaierie
Venete
CCM3
AM
Piracicaba
SAIL Ranchi Sideno
rBasau
ri
Mold taper Double
Parabolic
Simple
Parabolic
Triple Less friction
with
parabolic
Consta
nt
Primary
cooling
High better 6-8 l/kg High
better
Consta
nt
Mold flux Low visc.
better
1.48 dP
better than
2.5 dP1300
0,9 dP
1300oC
EMS Stronger
better
Roll gap Less bulging
with 0
instead of
0.3
Zone 1
cooling
Medium
better
0.3 l/kg total High
better
Consta
nt

29. Evolution during rolling
 Welding during rolling
 Unless they break to the
billet surface to become
oxidized during reheating
 Sulfides remain
 After rolling, a macroetching
on a transverse cut would
revealed the ghost lines, or
remaining sulfides
 Reagents revealing
segregation, like Oberhoffer,
Béchet-Beaujard, ingot
pattern, ammonium
persulfate, may be used
 Then, observation with
stereo microscope to identify
ghost lines, measuring crack
length in the transverse cut,
as well as distance to the
bar surface

30. Evolution during rolling
Assessment of ghost linesAssessment of ghost lines
On macroetched transverse cut of rolled bars,
according to its degree
Hot compression of a bar (simulating forging)

31. Conclusions
Off-corner cracks in billets and blooms are internal
defects refilled with manganese sulfides
Strand breakout during casting
Aligned manganese sulfides in rolled bar and wire rod
Defects in forging and heat treatment
Formation mechanism well understood since the early
eighties, after characterization by macroetching and
metallography, as well as solidification modelling
Many modifications in mold design include as aim a
minimization of off-corner cracks, in particular for high
casting speed

32. Conclusions
While the defect is present both in open and submerged
casting, troubleshooting reports mostly deal with
submerged casting, probably due to the influence of these
cracks in special bar quality rolled products
Mid carbon steels seems to be more prone to the defect.
Measures proposed to minimize its occurrence include
metallurgical measures as lower Mn/S ratio and low
superheat, as well as changes in mold design, mold flux of
lower viscosity, stronger primary cooling (and first zone of
secondary cooling), more intense electromagnetic stirring,
negative tolerance for foot rolls, and others

33. Jorge Madias
metallon
Full paper (not included in Proceedings): request to
jorge.madias@metallon.com.ar