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End splitting during rolling of long products: Billet quality or
rolling process? Jorge Madias
End splitting • Content
– Introduction
– Billet quality
– Rolling process
• Hot ductility
• Sulphur effect
• Temperature control
• Pass design
• Guiding issues
– Conclusions
Introduction • metallon: Technical services for the Latin
American steel industry & value chain
– Technical assistance (raw materials, ironmaking, steelmaking, rolling, product development)
– Open, in company and self-learning courses
– Met lab services
– Library services
– Articles for trade journals
• Brazilian references: Gerdau, ArcelorMittal Longos, Brasmetal Waelzholz, CSN, GrafTech, Minitec, Sinobras, Stollberg, SunCoke, Tecnosulfur, UNICAMP, Vetorial
Introduction • In long products
rolling, end splitting and/or central bursting may occur
• This happens for free-cutting steel, wire rod and shapes
Introduction • End splitting (alligatoring, split ends)
Introduction • Central bursting & cobble
Billet Quality • Rough cracks and ends quality
– Center cracks
– Diagonal cracks
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
Billet Quality • Center crack
– Billet with artificial crack in the end
– End splitting in the rolled bars
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
Billet Quality • End shape
– Pilot rolling mill trials, free cutting steels, 15 passes
– Rounded end cut with saw or machined with different shapes
Billet Quality • Not always end splitting origin can be traced
to billet defects
Rolling Process • Hot ductility
• Sulphur content and sulphur type
• Temperature control
• Pass design
• Guiding issues
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
Rolling Process • Hot ductility
– Carbon steels
0
20
40
60
80
100
800 900 1000 1100 1200 1300 1400
Temperatura °C
N°
de v
uel
tas a
la r
otu
ra
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
Rolling Process • Hot ductility
– Free cutting steels
Rolling Process • Sulphur effect
– The higher the sulphur, the lower the Mn/S ratio required to avoid FeS formation
Rolling Process • Sulphur effect
– Type II sulphides promote end splitting
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
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
Rolling mill • Temperature control
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
Rolling Process • Pass design
– Box-box pass with 35% reduction: overfilling and end splitting, in pilot rolling mill
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
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
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
Thank you for your attention!
Jorge Madias
San Nicolas, Buenos Aires, Argentina
www.metallon.com.ar
