A. S. Hamada - Manufacturing, mechanical properties and corrosion behaviour of hihg-Mn TWIP steels.pdf

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OULU 2007
C 281
ACTA
Atef Saad Hamada
U N I V E R S I T AT I S O U L U E N S I S
TECHNICA
C
MANUFACTURING,
MECHANICAL PROPERTIES
AND CORROSION
BEHAVIOUR OF
HIGH-Mn TWIP STEELS
FACULTY OF TECHNOLOGY,
DEPARTMENT OF MECHANICAL ENGINEERING,
UNIVERSITY OF OULU
ACTA UNIVERSITATIS OULUENSIS
C Te c h n i c a 2 8 1
ATEF SAAD HAMADA
MANUFACTURING,
MECHANICAL PROPERTIES AND
CORROSION BEHAVIOUR OF
HIGH-Mn TWIP STEELS
Academic dissertation to be presented, with the assent of
the Faculty of Technology of the University of Oulu, for
public defence in Auditorium TA105, Linnanmaa, on
October 19th, 2007, at 12 noon
O U L U N Y L I O P I S TO, O U L U 2 0 0 7
Copyright © 2007
Acta Univ. Oul. C 281, 2007
Supervised by
Professor Pentti Karjalainen
Reviewed by
Professor Javier Gil Sevillano
Doctor Pasi Peura
ISBN 978-951-42-8583-7 (Paperback)
ISBN 978-951-42-8584-4 (PDF)
http://herkules.oulu.fi/isbn9789514285844/
ISSN 0355-3213 (Printed)
ISSN 1796-2226 (Online)
http://herkules.oulu.fi/issn03553213/
Cover design
Raimo Ahonen
OULU UNIVERSITY PRESS
OULU 2007
Hamada, Atef Saad, Manufacturing, mechanical properties and corrosion behaviour
of high-Mn TWIP steels
Faculty of Technology, University of Oulu, P.O.Box 4000, FI-90014 University of Oulu, Finland,
Department of Mechanical Engineering, University of Oulu, P.O.Box 4200, FI-90014 University of
Oulu, Finland
Acta Univ. Oul. C 281, 2007
Oulu, Finland
Abstract
Austenitic high-Mn (15–30 wt.%) based twinning-induced plasticity (TWIP) steels provide great
potential in applications for structural components in the automotive industry, owing to their
excellent tensile strength-ductility property combination. In certain cases, these steels might also
substitute austenitic Cr-Ni stainless steels. The aim of this present work is to investigate the high-
temperature flow resistance, recrystallisation and the evolution of microstructure of high-Mn steels
by compression testing on a Gleeble simulator. The influence of Al alloying (0–8 wt.%) in the hot
rolling temperature range (800°C–1100°C) is studied in particular, but also some observations are
made regarding the influence of Cr alloying. Microstructures are examined in optical and electron
microscopes. The results are compared with corresponding properties of carbon and austenitic
stainless steels. In addition, the mechanical properties are studied briefly, using tension tests over the
temperature range from -80°C to 200°C. Finally, a preliminary study is conducted on the corrosion
behaviour of TWIP steels in two media, using the potentiodynamic polarization technique.
The results show that the flow stress level of high-Mn TWIP steels is considerably higher than that
of low-carbon steels and depends on the Al concentration up to 6 wt.%, while the structure is fully
austenitic at hot rolling temperatures. At higher Al contents, the flow stress level is reduced, due to
the presence of ferrite. The static recrystallisation kinetics is slower compared to that of carbon steels,
but it is faster than is typical of Nb-microalloyed or austenitic stainless steels. The high Mn content
is one reason for high flow stress as well as for slow softening. Al plays a minor role only; but in the
case of austenitic-ferritic structure, the softening of the ferrite phase occurs very rapidly, contributing
to overall faster softening. The high Mn content also retards considerably the onset of dynamic
recrystallisation, but the influence of Al is minor. Similarly, the contribution of Cr to the hot
deformation resistance and static and dynamic recrystallisation, is insignificant. The grain size
effectively becomes refined by the dynamic and static recrystallisation processes.
The tensile testing of TWIP steels revealed that the Al alloying and temperature have drastic
effects on the yield strength, tensile strength and elongation. The higher Al raises the yield strength
because of the solid solution strengthening. However, Al tends to increase the stacking fault energy
that affects strongly the deformation mechanism. In small concentrations, Al suppresses martensite
formation and enhances deformation twinning, leading to high tensile strength and good ductility.
However, with an increasing temperature, SFE increases, and consequently, the density of
deformation twins decreases and mechanical properties are impaired.
Corrosion testing indicated that Al alloying improves the corrosion resistance of high-Mn TWIP
steels. The addition of Cr is a further benefit for the passivation of these steels. The passive film that
formed on 8wt.% Al-6wt.%Cr steel was found to be even more stable than that on Type 304 steel in
5–50% HNO
3
solutions. A prolonged pre-treatment of the steel in the anodic passive regime created
a thick, protective and stable passive film that enhanced the corrosion resistance also in 3.5% NaCl
solution.
Keywords:
Al alloying, anodic passivation, ductility, flow stress, high-Mn steels, hot
deformation, mechanical twinning, passive film, recrystallization, stacking fault energy,
strength

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