Triple-phase high-strength high formability steel

Directorate-General For Research And Innovation European Commission

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 16
• EAN13: 928284658
• Langue: English
• Poids de l'ouvrage: 8 Mo

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EURO PEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
Mechanical working (rolling)
Triple-phase
high-strength high
formability steels
h
Report
STEEL RESEARCH EUR 18471 EN EUROPEAN COMMISSION
Edith CRESSON, Member of the Commission
responsible for research, innovation, education, training and youth
DG XII/C.2 — RTD actions: Industrial and materials technologies —
Materials and steel
Contact: Mr H. J.-L. Martin
Address: European Commission, rue de la Loi 200 (MO 75 1/10),
B-1049 Brussels — Tel. [phone]; fax [phone] European Commission
Mechanical working (rolling)
Triple-phase high-strength
high formability steel
P. Polatidis, A. Stamou
Mirtee SA
A' Industrial Area of Volos
GR-38500 Volos
Contract No 7210-EC/701
1 December 1990 to 30 November 1993
Final report
Directorate-General
Science, Research and Development
EUR 18471 EN 1998 LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-4658-X
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER CONTENTS
PAGE
1. INTRODUCTION 5
2. BACKGROUND
3. EXPERIMENTAL PROCEDURE g
3.1 Material Fabrication 9
3.2 Heat treatments
3.2.1 Bainitic Transformation Route
3.2.2 Carbide Conversion Route
3.2.2.1 Simulation of Intercritical Annealing
in a Dilatometer
3.3 Mechanical properties evaluation 10
3.4 Microstructural characterisation1
3.4.1 Optical Microscopy
3.4.2 Transmission electron microscopy
34.3 Scanning electrony
3.4.4 X-ray Diffractometry
3.4.5 STEM microanalysis
3.5 Planar anisotropy measurements (R value) 12
4. RESULTS 13
4.1 Thermodynamic Calculations 1
4.1.1 Transformation temperatures
4.1.2 Phase equilibria
4.2 Bainiticn Route4
4.2.1 Mechanical Properties
4.2.2 Microstructural Characterisation
4.2.3 Measurement of Retained Austenite content
4.2.4 Calculation of R value
4.2.5 Discussion
4.3 Carbide Conversion Route 17
4.3.1 Simulation of Intercritical Annealing in the Dilatometer
4.3.2 Microstructural characterisation
4.3.3 STEM microanalysis
4.3.4 Mechanical Properties
4.3.5 Discussion
5. CONCLUSIONS 20
6. REFERENCES3
7. TABLES5
8. FIGURES 3
9. LIST OF TABLES 8
10. LIST OF FIGURES1. INTRODUCTION
The aim of the research project is to investigate the development a new class of steels,
namely triple-phase steels, with strength and formability combinotions superior to HSLA and
duol - phase steels.
Targeted applications ore the stretch forming and other cold forming operations where high
formability at high strength levels is reguired.
These gpplications are widely found in the automotive industry but other industrial sectors
ore concerned with them os well.
The central idea is to form a metastable austenite dispersion (the third phase) in a two -
phase matrix which will be either ferrite / martensite or ferrite / bainite depending on the
heat treatment route followed.
The austenite stability will be controlled in such a way, that the austenite present will
transform to martensite only under the mechanical stresses acting during the stretch forming
operation. The transformation induced plasticity effect (TRIP) of the dispersed austenite
should enhance the ductillity and formability. High strength values should be attained by
controlling the martensite or boinite gmount and distribution, while the C level will be retained
to low levels for acceptable weldability.
The property objectives were set to a 3% flow strength of 500-700 MPa and a uniform
elongation 20-25% in uniaxial tension.
A comprehensive literature review on current developments in the subject is given in the
Background section that follows.
BACKGROUND
The major criteria in the process of development of advanced materials to meet the latest
automotive fuel economy standards are strength - ductility improvements associated with cost
- effectiveness.
HSLA as well as Dual Phase Steels have been designed to provide the best substitutes for
low carbon sheet steel for forming gpplications. Although microolloyed steels initially
emerged as strong candidgtes for most applications, their increased strength level was only
ochieved with a consideroble loss in ductility ond formability (1 ). Dual phase steels were thus introduced to provide the necessary stretch - formability
properties to overcome the deficiency of HSLA steels. Initial studies indicated that the
formability of these steels was superior to that of even the lower strength microolloyed
steels ond gpproached the formability of mild steels. In addition, the transformation
strengthering mechanism employed in these steels has made the strength / formability
balgnce very sensitive to the steel corbon content. Consequently, there is some loss of
property reproducibility ond the improved formobility offered by dual - phase steels cannot
be fully utilised.
The term dual phase refers to the presence of essentially two phases, ferrite and martensite
in the microstructure, although small amounts of bainite, pearlite gnd retgined gustenite may
also be present.
Their high work-hardering rate results in g yield strength of 550 MPo gfter only 3-5%
deformotion ond g formobility level equivglent to that of much lower strength sheet steels.
The effect of dual phase microstructure on uniform and total elongation is influenced by
many factors such as the volume fraction of martensite, cgrbon content, plosticity gnd
distribution of the martensite, alloy content of the ferrite and retained austenite.
Extended research work has been concentrated on the understanding of ductility
improvements observed in ferrite - martensite steels.
Davies (2) and Araki et gl (3) showed thot the uniform elonggtion decregses in g non-lineor
mgnner with increasing percent martensite, while Speich and Miller (4) found additionally that
the uniform elongation increases slightly when the carbon content of the martensite phase is
decreased. Rashid (5) has argued that the improved ductility of ferrite -e steels, in
contrast to ferrite - pearlite steels, is caused by the higher ductility of the martensite phase.
Distribution of the martensite phase must also influence ductility, but very few studies of this
variable have been reported, except for the work of Becker and Hornbogen (6). For any
given percentage of martensite a set of widely spaced, small martensite particles is desired.
Because the distribution of the martensite phase is determined by the nucleation of austenite
particles of the cementite or pearlite phases present in the starting microstructure, it is
important, in intercritically annealed dual phase steels that this microstructure be as uniform
and fine as possible.
Lowering of the carbon content of the ferrite phase by decregsing the cooling rote gfter
continuous onnegling hgs glso been reported by a number of investigators to be critical in
obtaining the highest possible ductility in dual - phase steels. In pgrticulgr, Hgygmi et gl (7)
reported that the ductility obtained after water quenching is much inferior to that obtained
with a mild cooling rate because of the lower carbon content of the ferrite in the slower
cooled material. Krauss and Matlock (8) argued that increasing the amount of epitaxial ferrite formed upon
cooling has gn improving effect on ductility. This wos related to the low carbon content of
the epitaxial ferrite interface which maintains equilibrium with the austenite phase into which
it is growing.
The phenomenon of transformotion of retoined gustenite during plastic strgining gnd the
resultgnt incregse in work - hordening rgte hgs also been used to explain the higher ductility
of dual - phase steels. This is similar to the well known tronsformotion - induced plosticity
mechanism, (TRIP), used to explain the high ductility of metastable austenitic stainless steels
(9).
Furukawa et al (10) showed thot the retgined gustenite content incregsed from 2% to 9%
when intercritically annealed dual phase steels were cooled to successively lower
temperatures and then quenched.
Although the strength decreased simultaneously and a ductility increase would be expected
from this effect alone, Furukawa et al argued that the ductilitye was due to the
increased retained austenite content. The increase of retained austenite with lower
quenching temperatures (or lower cooling rates) is presumably attributed to stabilisation
effects.
Marder (11), Rigsbee and Vander Avend (12) as well as Rgshid gnd Rgo (13), olso suggested
a close connection between the amount of retained austenite and the ductility of dual phase
steels.
In contrast, Eldis (14) found n