Strain rate sensitivity of automotive sheet steels: influence of plastic strain, strain rate, temperature, microstructure, bake hardening and pre-strain [Elektronische Ressource] / vorgelegt von Patrick Larour

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2010
• Nombre de lectures: 36
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Strain rate sensitivity of automotive sheet steels: influence of plastic strain,
strain rate, temperature, microstructure, bake hardening and pre-strain
Von der Fakultät für Georessourcen und Materialtechnik der
Rheinisch -Westfälischen Technischen Hochschule Aachen
zur Erlangung des akademischen Grades eines
Doktors der Ingenieurwissenschaften
genehmigte Dissertation
vorgelegt von Dipl.-Ing.
Patrick Larour
aus Villeneuve la Garenne, Paris
Berichter: Univ.-Prof. Dr.-Ing. Wolfgang Bleck
Professor Dr.-Ing. Dierk Raabe
Tag der mündlichen Prüfung: 22. April 2010Berichte aus dem IEHKInstitut für Eisenhüttenkunde
RWTH Aachen
Patrick Larour
Strain rate sensitivity of automotive sheet steels:
influence of plastic strain, strain rate, temperature,
microstructure, bake hardening and pre-strain
Herausgeber:
Prof. Dr.-Ing. W. Bleck
Prof. Dr.rer.nat. Dr.-Ing.e.h. W. Dahl
Prof. Dr.-Ing. T. El Gammal
Prof. Dr.-Ing. H.W. Gudenau
Prof. Dr.-Ing. D. Senk
Band 1/2010
Shaker VerlagBibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche
Nationalbibliografie; detailed bibliographic data are available in the Internet at
http://dnb.d-nb.de.
Zugl.: D 82 (Diss. RWTH Aachen University, 2010)
Copyright Shaker Verlag 2010
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This thesis was conducted during my employment as Research Engineer at the Institute of Ferrous
Metallurgy, RWTH Aachen.
I am extremely thankful to my supervisor Univ.-Prof. Dr.-Ing. W. Bleck, for the scientific guidance,
active support and interest along my thesis work. Special thanks also to Prof. Dr.-Ing. habil.
D. Raabe for taking over the co-referee, and to Univ.-Prof. Dr.-Ing. Dieter Senk, chairman of the
doctoral thesis examination.
Dr. P. Splinter, Dr. G. Heßling as well as G. Leisten are gratefully acknowledged for their project
management assistance and technical advices.
I express sincere appreciations to all my colleagues, especially Dr. I. Schael, Dr. T. Böllinghaus,
Dr. A. Frehn, Dr. A. Bäumer, Dr. E. Ratte, Dr. S. Papaefthymiou, S. Brühl, K. Dahmen, S.
Hoffmann and T. Labudde, for many interesting discussions, encouragements and the excellent
team work.
The help of H. Majedi in the development of dynamic analysis software is also gratefully
acknowledged.
I would like to thank especially my office colleagues Dr. I. Kim, Dr. M. Shehata and Dr. M. Naderi
for the nice working athmosphere.
Special thanks for the technical assistance and broad experimental know-how of W. Schümmer,
D. Sodar and H. Tschammer in the challenging field of servohydraulic dynamic testing.
Many thanks also to J. Noack for his excellent student work on dynamic tensile test vibration
behaviour, as well as B. Landry Freuze, S. Konovalov and A. Mousavi for their valuable help in
dynamic testing analysis.
Most of the presented results arised from two consecutive projects, which were co-ordinated by the
Steel Institute VDEH and financially supported by the german steel and automotive industry. The
author expresses his gratitude to the Steel Institute VDEH and the sponsor companies.
This work has been also partly performed with a financial grant from the European Coal and Steel
community (ECSC project 7210-PR/369), which is gratefully acknowledged for its financial
support. All consortium partners are gratefully acknowledged..
Other financial support to be acknowledged was a grant under the program Procope founded by
EGIDE/DAAD French-German exchange program. Many thanks to Professor J.R. Klepaczko and
Professor A. Rusinek from Metz University for the intensive scientific cooperation and common
publications.
Finally I would like to express my most sincere thanks to my wife Maria and my daughter Elisabeth
for their endless patience and encouragement.
Linz in April 2010Abstract
This experimental work shows the different parameters influencing the strain rate sensitivity
behaviour of automotive sheet steel grades in crash conditions. Most investigations have been
-3 -1performed in the strain rate range [10 -200s ] and temperature range [233-373K] with
3 -1
servohydraulic tensile testing machines. Additional Split-Hopkinson bar testing results up to 10 s
have also been included at room temperature. The focus has been laid on the “apparent” strain rate
sensitivity, determined based on multiple dynamic flow curves at constant strain rate in a semi-
logarithmic ( values) or logarithmic (m values) way.10
It has been shown that the dynamic behaviour in the investigated strain rate and temperature range
is clearly thermally activated for a wide range of automotive sheet steels. This means that an
increase in strain rate is nearly equivalent to a decrease in temperature. The strain rate sensitivity
values are dependent on the strain rate range considered, as well as on the temperature and plastic
deformation range chosen. The strain rate sensitivity decreases with increasing plastic strain level
due to a gradual exhausting of work hardening potential combined with adiabatic softening effects.
The strain rate sensitivity increases with decreasing temperature or increasing strain rate, which is
often omitted when considering literature data.
The strain rate sensitivity is also dependent on the microstructure investigated. The strain rate
sensitivity decreases strongly with increasing strength level, especially below 400MPa yield
strength or 500MPa tensile strength, and is stabilised at a low level for AHSS and UHSS steel
grades. The strain rate sensitivity decreases for single phase ferritic mild, HSS and HSLA steel
grades, mainly due to solid solution alloying with Mn, Si and P elements. Long range mechanisms
such as precipitation hardening, grain refinement or cold work do not influence the strain rate
sensitivity. With increasing second hard phase content, the strain rate sensitivity decreases due to
the decrease of relative volume content of strain rate sensitive ferrite in multiphase steels.
The TRIP effect decreases the strain rate sensitivity in low alloy TRIP steels in comparison to
dualphase steels. High alloy TRIP steels show some negative strain rate sensitivity in the low strain
rate and high strain range. A significant decrease in the TRIP effect intensity is seen at strain rates
-1
above 1s at high strain level, which is related to an adiabatic temperature increase.
Uniaxial, plane strain or biaxial pre-straining up to around 0,10 equivalent strain does not influence
the strain rate sensitivity of sheet steels in comparison to the as-delivered material. A bake
hardening heat treatment with or without pre-straining does not influence the strain rate sensitivity
significantly neither. Forming and/or bake hardening does not affect particularly the subsequent
strain rate sensitivity in crash conditions. Cold work or bake hardening introduces obstacles to
dislocations, which are rather of athermal nature, so that the strain rate sensitivity is not influenced.
For high alloy TRIP steels, the magnitude of subsequent TRIP effect is increased with increasing
pre-straining level, which slows down adiabatic stress softening quite effectively.
This experimental work allows some reliable comparisons between different alloying concepts and
helps to identify the parameters influencing effectively the strain rate sensitivity, such as strain rate,
temperature, plastic deformation, solid solution alloying, second phase hardening and TRIP effect.
This work delivers additionally a wide database for strain rate sensitivity values of automotive sheet
steel grades, which can be referred to for further experimental and modelling investigations.Kurzfassung
Diese experimentelle Arbeit stellt die wichtigsten Einflussparameter auf die
-3 -1
Dehnratenempfindlichkeit von industriellen Karosseriestählen im Dehnratenbereich [10 -200s ]
und Temperaturbereich [233-373K] mit servohydraulischen Schnellzerreißzugprüfmaschinen dar.
3 -1
Einige zusätzliche Split-Hokinson bar Prüfungen bis 10 s bei Raumtemperatur sind ebenfalls
miteinbezogen. Die Dehnratenempfindlichkeit wurde auf der Basis von dynamischen Fließkurven
bei steigenden Dehnraten ermittelt. Die halb-logarithmische und logarithmische m10
Dehnratenempfindlichkeit wurde dabei berechnet.
Das dynamische Verformungsverhalten wird mit dem Modell des thermisch aktivierten Fließens
zuverlässig widergespiegelt. Dabei wirkt sich eine Erhöhung der Dehnrate ähnlich wie eine
Abnahme der Temperatur auf die dynamische Fließspannung aus.
Die Dehnratenempfindlichkeit von Stählen weist eine starke Abhängigkeit vom betrachteten
Dehnratenbereich, sowie von der Temperatur und plastischen Dehnung auf. Die
Dehnratenempfindlichkeit nimmt mit steigender plastischer Dehnung ab. Dies ist sowoh