Évolution microstructurale d'un acier Dual Phase. Optimisation de la résistance à l'endommagement, Microstructural evolution of Dual Phase steel. Improvement of damage resistance

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Institut National Polytechnique de Lorraine


Ecole des Mines de Nancy


Ecole doctorale Energie, Mécanique et Matériaux (ED409)


Laboratoire de Science et Génie des Surfaces – UMR CNRS 7570

Docteur de l’INPL

Science et Ingénierie des Matériaux

Irina PUSHKAREVA


Evolution microstructurale d’un acier Dual Phase.
Optimisation de la résistance à l’endommagement.


Thèse dirigée par Abdelkrim REDJAÏMIA


Soutenue publiquement le 13 Novembre 2009 devant la commission d’examen
Jury :


Anna Fraczkiewicz Directeur de recherche, ENSM St Etienne Rapporteur

Alexandre Legris Professeur, UST Lille

Sabine Denis Professeur, Nancy-Université-INPL Examinateur

Mohamed Gouné Ingénieur, ArcelorMittal, Maizières-lès-Metz Examinateur

Antoine Moulin Ingénieur, ArcelorMittal, Maizières-lès-Metz Examinateur

Abdelkrim Redjaïmia Professeur, Nancy-Université-INPL Examinateur

iAcknowledgments

The present work is a result of collaboration between ArcelorMittal R&D center, Maizières-
lès-Metz and National School of Mines of Nancy, France. The experiments were carried out
at the ArcelorMittal R&D center Maizières-lès-Metz, except where indicated. I am indebted
to ArcelorMittal group for financial support of this project.

I am grateful to my university supervisor A. Redjaïmia for advice and useful comments,
especially during the reviewing of this manuscript.

I would like to express my thanks to my industrial supervisor A. Moulin for his great
encouragement and support throughout this work.

I acknowledge the support, help and interest that I receive from G. Metauer.

I am grateful to O. Bouaziz, S. Allain and C. Scott for many inspiring discussions on the
subject of the damage behaviour of the steels. I am also grateful to C. Scott who carefully
reviewed the script and made very useful comments. In addition, I acknowledge C. Scott for
EELS measurements and TEM observations.

I would like to express my appreciation to M. Gouné for numerous discussions which
contributed to the development of this work and permitted understanding the experimental
observations.

I wish to thank J. Drillet for so many advices in the microstructural characterization.

I would like to thank C. Landron for in-situ tensile test data.

All the help from N. Valle with the NanoSIMS characterization is gratefully acknowledged.

I would like to thank A. Perlade and S. Cobo for useful suggestions.

I would like to thank the Documentation department staff for help in the literature research
and particularly S. Fogel.

I would like to thank the technicians of the Auto Center for their kind help with the
experimental work.

I would like to express my appreciation to all the members of the Auto Center for their help
and friendship.

I would like to thank A. Fraczkiewicz and A. Legris for accepting to judge my PhD work.
Other members of jury are also gratefully acknowledged.


ii















In everything I seek to grasp
The fundamental:
The daily choice, the daily task,
The sentimental.

To plumb the essence of the past,
The first foundations,
The crux, the roots, the inmost hearts,
*
The explanations.

Boris Pasternak, 1956


* Translation from Russian www.friends-partners.org
iii
Table of contents

Introduction .............................................................................................................................. 9
Literature review.................................................................................................................... 13
I.1 Dual Phase steel microstructure formation..................................................................... 13
I.1.1 Austenite formation during intercritical annealing.................................................. 13
I.1.2 Transformation of austenite after intercritical annealing......................................... 16
I.1.3 Changes in ferrite phase during intercritical annealing and cooling........................ 17
I.1.4 Dual Phase steel microstructure............................................................................... 18
I.2 Martensite structure ........................................................................................................ 20
I.2.1 Martensitic transformation....................................................................................... 20
I.2.2 Martensite morphology............................................................................................ 21
I.3 The effect of the alloying elements................................................................................. 24
I.3.1 Influence of alloying elements on Continuous Cooling Transformation (CCT)
diagram............................................................................................................................. 25
I.3.2 The role of different alloying elements.................................................................... 26
I.3.3 The effects of alloying elements on austenitising.................................................... 27
I.3.4 The effects of alloying elements on ferrite formation ............................................. 28
I.3.5 The effects of alloying elements on martensite formation....................................... 28
I.3.6 Segregations in Ingots and Castings ........................................................................ 29
I.4 Tempering....................................................................................................................... 31
I.4.1 Tempering of ferrous martensites ............................................................................ 31
I.4.2 Stages of tempering.................................................................................................. 31
I.4.3 Tempering reactions in DP steels 36
I.5 The DP steel deformation behaviour .............................................................................. 39
I.5.1 Mechanical behaviour.............................................................................................. 39
I.5.2 Continuous yielding behaviour................................................................................ 40
I.5.3 Tensile strength........................................................................................................ 41
I.5.4 Ductility ................................................................................................................... 42
I.6 The damage mechanisms in DP steel during the ductile fracture process...................... 43
I.6.1 Void nucleation 43
I.6.2 Void growth ............................................................................................................. 45
I.6.3 Void coalescence ..................................................................................................... 45
4
I.7 Microscopic fracture appearance in DP steel.................................................................. 45
I.8 Damage resistance of DP steel through Hole Expansion (HE)....................................... 46
Microstructures and mechanical properties........................................................................ 47
II.A Microstructure formation.............................................................................................. 47
II.A.1 Chemical composition and initial microstructures .................................................... 47
II.A.2 Continuous Cooling Transformation (CCT) diagram for studied DP steel............... 48
II.A.3 Determination of intercritical region temperatures 49
II.A.4 Heat treatments .......................................................................................................... 50
II.A.4.1 Thermal treatment cycles.................................................................................... 50
II.A.4.2 Direct quenching..