End splitting occurs because the material being rolled has not enough ductility to withstand the stress to which it is submitted. This may happens for different reasons. Coarse cracks in the billet end, like central or diagonal cracks, weakens the end, particularly when the plane where are located coincides in part with the symmetry plane between rolls. Hot ductility of steel depends on the one hand of their intrinsic features, and on the other hand, on the temperature at which they suffer the stress, and its speed. It is important to roll the steel within the range of higher ductility at a given deformation speed. This is more critical for steels with inherent low ductility as those containing high sulfur. The role of MnS stringers is also clear; having S controlled at the lower level of the specification is favorable. Nevertheless it is worth to mention that if caster condition is proper and excessive thermal/mechanical stresses do not arise, very high Mn/S ratio is not necessary. Bar ends loss temperature faster. Another factor is roll cooling, it has to be correctly oriented, not excessive and keeping the position along the processing time. In other factors experimental and modeling results are apparently controversial. There is coincidence in the fact that more friction between bar and rolls promotes splitting, but not in factors like roll diameter and reduction. On the basis of pilot rolling results, plane, box-box, square to round and oval to round passes are favorable to avoid splitting, while oval to square promotes splitting.

1. End splitting during rolling of
long products: Billet quality or
rolling process?
Jorge Madias

2. End splitting
• Content
– Introduction
– Billet quality
– Rolling process
• Hot ductility
• Sulphur effect
• Temperature control
• Pass design
• Guiding issues
– Conclusions

3. Introduction
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4. Introduction
• In long products
rolling, end splitting
and/or central
bursting may occur
• This happens for free-
cutting steel, wire rod
and shapes

5. Introduction
• End splitting (alligatoring, split ends)

6. Introduction
• Central bursting & cobble

7. Billet Quality
• Rough cracks and ends quality
– Center cracks
– Diagonal cracks

8. Billet Quality
• Center crack
– Influence on ends quality and end splitting
High susceptibility to end
splitting (cut altered by center
crack)
Medium susceptibility to
end splitting
Low susceptibility to end
splitting

9. Billet Quality
• Center crack
– Billet with artificial crack in the end
– End splitting in the rolled bars

10. Billet Quality
• Diagonal crack
– Usually associated to
rhomboidity
– Initiated in off corner
crack
– In the pass where it
occurs, it must be
some coincidence
between the crack
plane and the gap
between rolls

11. Billet Quality
• End shape
– Pilot rolling mill trials,
free cutting steels, 15
passes
– Rounded end cut
with saw or machined
with different shapes

12. Billet Quality
• Not always end splitting origin can be traced
to billet defects

13. Rolling Process
• Hot ductility
• Sulphur content and sulphur type
• Temperature control
• Pass design
• Guiding issues

14. Rolling Process
• Hot ductility
– Measurement by
torsion, tension or
compression test, at a
given deformation
rate and
temperature, on a
machined sample
from a defined part of
the billet

15. Rolling Process
• Hot ductility
– Carbon steels
0
20
40
60
80
100
800 900 1000 1100 1200 1300 1400
Temperatura °C
N°devueltasalarotura
10 6 0
10 8 2 B 3
10 70
10 4 0
10 2 0
10 10
Po linó mic a ( 10 8 2
B 3 )
LTC

16. Rolling Process
• Hot ductility
– Free cutting steels

17. Rolling Process
• Sulphur effect
– The higher the
sulphur, the lower the
Mn/S ratio required
to avoid FeS
formation

18. Rolling Process
• Sulphur effect
– Type II sulphides promote end splitting

19. Rolling Process
• Sulphur effect
– Tests in pilot rolling
mill, with free-cutting
leaded and non-
leaded bars
– The lower the sulphur
content, the larger the
number of passes
without end splitting

20. Rolling Process
• Temperature control
– Hot ductility curves show clearly the need to work
within a defined temperature range, for each steel
– Large temperature drop must be avoided (for
example, in relation with roll cooling)
– For free-cutting steel, an auxiliary burner for billet
end reheating have been used, as well as
induction heaters in intermediate positions along
the rolling mill

21. Rolling mill
• Temperature control

22. Rolling Process
• Pass design
– More propension to split ends for
• Higher reduction
• More friction between bar and rolls
• Oval to square pass design
– Less propension to end splitting
• Flat passes
• Box-box
• Square to round
• Oval to round

23. Rolling Process
• Pass design
– Box-box pass with 35% reduction: overfilling and
end splitting, in pilot rolling mill

24. Rolling Process
• Guiding issues
– Twisting
• For instance, when in a square pass, if the end does not
enter in plane position but in a diagonal, more
deformation is locally applied
• Then, after other passes, end splitting may arise

25. Conclusions
• End splitting occurs when the bar has not
enough ductility to support the mechanical
efforts at which it is submitted
• Common carbons steels should have enough
ductility to be rolled without splitting, except
if rough diagonal or center cracks are present
of if high temperature drop occurs

26. Conclusions
• For steels with inherent low ductility, another
factors must be taken into account
– Rolling within best ductility range
– Keeping temperature high during rolling
– Avoiding sensitive pass design
– Avoiding too high reduction in a given pass

27. Thank you for your attention!
Jorge Madias
San Nicolas, Buenos Aires, Argentina
jorge.madias@metallon.com.ar
www.metallon.com.ar