End splitting during long products rolling billet quality of rolling process

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

jorge.madias@metallon.com.ar

www.metallon.com.ar