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

Irina Pushkareva

Description

Sous la direction de Abdelkrim Redjaïmia
Thèse soutenue le 13 novembre 2009: INPL
Actuellement, l’industrie automobile est à la recherche d’une meilleure solution pour l’allégement de la structure de véhicule afin de diminuer la consommation de carburant et par conséquent diminuer les émissions nocives de CO2. Les aciers à très haute résistance (THR) mécanique permettent d’obtenir les tôles d’acier à section diminué avec les mêmes ou meilleurs propriétés fonctionnels. Les aciers Dual-Phase (DP), constitués majoritairement d’une phase ductile, la ferrite, et d’une phase dure, la martensite, occupent une place importante en tant que matériaux de structure destinés au challenge préoccupant l’industrie automobile. Une bonne résistance à l’endommagement est exigée pour leur utilisation en tant que des pièces de structures et de renfort pour l’automobile. Il a été bien établi que la résistance à l’endommagement des ces aciers Dual-Phase est contrôlée par leur microstructure. Ce travail de thèse s’est inscrit dans une logique de compréhension des mécanismes d’endommagement d’un acier Dual-Phase modèle, le DP 780, en fonction de différents paramètres microstructuraux. Deux mécanismes d’endommagement ont été identifiés pour l’acier DP 780 : la décohésion de l’interface ferrite/martensite et la formation de cavités autour des carbures, dans la martensite revenue. Un modèle qualitatif de mécanisme d’endommagement a été développé afin de pouvoir prédire l’endommagement de l’acier DP 780. Ce modèle qualitatif, développé pour l’acier DP 780, servira de base d’approfondissement de modèles plus élaborés et quantitatifs permettant la compréhension et la prédiction de l’endommagement des aciers Dual-Phase, de façon générale
-Aciers à très haute résistance mécanique
-NanoSIMS
-Eels
-Concentration locale en carbone
-Décohésion de l’interface
-Interface carbure/martensite revenu
-Endommagement
-Microstructure
-Dual-Phase
In the automotive industry current environmental concerns require that the vehicle fuel consumption and CO2 emissions should be reduced as much as possible. It is therefore advantageous to reduce the weight of body in white components by replacing existing parts with higher strength, thinner gauge alternatives with equivalent or improved functional properties. Dual Phase (DP) steels are a class of high-strength low-alloy steels characterized by a microstructure consisting of martensite and ferrite. Dual Phase steels combine high strength levels with good ductility. Thus, DP steels are potentially very attractive for the automobile industry. In addition to the required high strength and ductility, DP steel has to be cold formed into complex shapes. It appears that DP steel damage behaviour is very complex and cannot be predicted using existing models based on standard mechanical properties. This work is concerned with the study of microstructural evolution and investigation of the relation between the microstructure and damage mechanisms in a reference DP 780 steel. Two damage mechanisms have been identified in this DP steel: ferrite/martensite interface decohesion and void formation at tempered carbides. A simple modeling for qualitative description of the observed damage formation mechanisms is proposed. This modeling permits a basic understanding of the experimentally observed trends and could be used as the starting point for a more detailed analysis in future
-High Strength Steels
-NanoSIMS
-Eels
-Local carbon concentration
-Dual-Phase
-Microstructure
-Damage
-Ferrite/martensite interface
-Carbide/tempered martensite interface
-Interface decohesion
Source: http://www.theses.fr/2009INPL084N/document

Sujets

• Eels
• Endommagement
• Microstructure
• Fatale (film)

Informations

• Publié par: Thesee
• Nombre de lectures: 343
• Langue: English
• Poids de l'ouvrage: 12 Mo

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LIENS




Code de la propriété intellectuelle. Articles L 122.4 e la propriété intellectuelle. Articles L 335.2 – L 335.10
http://www.cfcopies.com/V2/leg/leg_droi.php
http://www.culture.gouv.fr/culture/infos-pratiques/droits/protection.htm
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..