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<p>Bainite is a low temperature transformation product of austenite decomposition in steels. Its unique range of microstructures offers promising combinations of strength with ductility. At low transformation temperatures the crystallography of a phase transformation often plays an important role in the overall microstructure and how it develops. Therefore in this study the structures of ferrite/cementite and ferrite/austenite interfaces in bainite were investigated from a crystallographic viewpoint. After describing these interfaces, the idea of interphase boundary nucleation of cementite on a ferrite/austenite interface was investigated.</p> <p>An O-line model (a special case of the O-Lattice) was used to explain the observed experimental results on orientation relationship, habit plane and good matching direction between ferrite and cementite. The calculated orientation relationship was used in an NCS (near coincident site) model to describe several possible edge facets of cementite precipitates. The major observed edge facet in cementite is deviated from the more favored interfaces based on the NCS model. This deviation could imply that the edge facets are non-equilibrium interfaces whose orientations and morphologies are kinetically determined.</p> <p>Focused Ion Beam sectioning, conventional transmission electron and optical microscopy were used to shed more light on the three dimensional nature of a complex cementite-free bainitic microstructure. The faceted interfaces of bainitic ferrite were characterized and it was shown that the habit plane contains edge misfit dislocations. The orientation of the bainitic ferrite lath did not match an O-line model. Transformation time was considered to play an important role on the orientation and morphology of the bainitic laths and interfacial dislocation character.</p> <p>Finally, with the aid of known crystallographic relations and interfaces between the ferrite/cementite, ferrite/austenite and austenite/cementite phases, a model for cementite nucleation was proposed. This interphase boundary nucleus is assumed to form on a coherent ferrite/austenite interface and to possess ferrite/cementite and austenite/cementite calculated habit planes as two main facets surrounding the nucleus. It was shown that cementite nucleation would be viable if interfacial energies of all surrounding facets of a nucleus are in a semi-coherent energy range.</p> / Doctor of Philosophy (PhD)

The development of high strength hot rolled steels is an important area for improving vehicle fuel efficiency. In collaboration with ArcelorMittal, this project focussed on developing a hot rolled steel with 980 MPa ultimate tensile strength, 800 MPa yield strength and 50% hole expansion ratio. To achieve the target mechanical properties, four different chemistries were trialled which varied the carbon, niobium and vanadium contents. Six combinations of finishing, coiling and intermediate temperatures were trialled for each chemistry. The effects of thermomechanical processing parameters and alloying contents on the mechanical properties were determined through tensile and hole expansion testing. Microstructural analysis was completed to correlate the mechanical properties to the microstructural characteristics. Microscopy techniques performed included optical microscopy, scanning electron microscopy, transmission electron microscopy and atom probe tomography. The phase transformations which occur during thermomechanical processing were investigated using dilatometry testing. Microstructural characterization was used to determine the breakdown of strengthening contributions from intrinsic, solid solution, grain boundary, precipitation and dislocation strengthening. Trials varying the processing parameters and steel chemistry led to an understanding of how thermomechanical processing and alloying influence the microstructural features and corresponding mechanical properties in hot rolled microalloyed steels. / Thesis / Master of Applied Science (MASc)

Ce travail de thèse concerne l’acier 16MND5 des cuves des Réacteurs à Eau Préssurisée (REP).L’étude vise à caractériser de manière quantitative la microstructure et à comprendre l’effet desparamètres influents (vitesse de refroidissement, température de revenu …) sur sa formation. D’unpoint de vue expérimental, l’étude est menée grâce à l’utilisation des techniques d’observationcomme la métallographie, le MET, MEB et l’EBSD, et la réalisation de traitements thermiquescontrôlés (dilatométrie notamment).Ces observations visent à identifier les différents constituants de l’acier et de déterminer lesgrandeurs quantitatives les caractériser (tailles, fractions volumiques des différentes phases ainsique leurs orientations cristallographiques).Nous avons dans un premier temps caractérisé de manière complète et quantitative, lamicrostructure de l’acier 16MND5 à l’état de réception (trempé et revenu). L’étude a permis demontrer qu’il existe un gradient de microstructure entre la peau interne et le coeur de l’épaisseur dela virole. La microstructure à coeur est constituée de ferrite proeutectoïde, de bainite et d’amas decarbures, tandis qu’en peau interne on est en présence de bainite et d’amas de carbures.Une campagne d’essais de dilatométrie a été réalisée afin de simuler la microstructure brute detrempe de l’acier 16MND5 et d’observer l’effet de la vitesse de refroidissement, des températuresde transformation sur la microstructure des aciers de cuve. Ces essais ont révélé que lamicrostructure de l’acier 16MND5 après refroidissement est constituée de bainite, d’îlots demartensite et d’austénite résiduelle (M-A) enrichie en carbone. Ce sont ces îlots M-A qui donnentnaissance aux amas de carbures lors du revenu final.L’effet de la composition chimique de cet acier a été étudié grâce à l’utilisation d’une tôle ayant unecomposition chimique plus riche en carbone et en éléments d’alliages. Ce travail a montré quel’augmentation des teneurs en carbone et en éléments d’alliages modifiaient la morphologie et lacristallographie de la ferrite.L’étude des comportements mécaniques des constituants présents en peau interne, quart épaisseur etmi épaisseur a été réalisée grâce à des essais de résilience sur des éprouvettes de mini charpy. Cetteétude a permis de montrer qu’à basse température (-120°C) la peau interne présente de meilleurespropriétés de résilience que le quart et la mi épaisseur de la virole. Des examens de la surfacelatérale des éprouvettes de Charpy ont montré que la présence de ferrite proeutectoïde favorise lagermination de micro-fissures de clivage. / Reactor pressure vessel is the second security barrier of the nuclear reactor and it is elaboratedwith a low carbon steel ( C-0.16%). Due to the large size of the ferrule there is temperatureand cooling rate gradient, which lead to microstructure gradient. To develop predictive modelit is necessary to well describe the microstructure which depend to the processing parameters.We focuse our work in investigating the effect of the processing parameters (cooling rate,chemical composition...) on the final microstructure during phase transformation.To get these informations and better describe the microstructure, we have used someexperimental technics like SEM, EBSD, TEM and Optical Microscopy. The steel used is alow carbon steel with a composition of C-0.16%, Mn-1.32%, Ni-0.72%, Mo-0.49%, Si-0.23%, Cr-0.23%, P-0.010%, S-0.004%. The material has been tempered in the range 635°C-660°C after cooling. Three positions have been chosen for examinations, because the coolingrate is not the same between the center and the edge of the material. The results of theobservations made in the different scale, indicate that the microstructure is mainly baniticwith bainitic ferrite and cementite precipitates. Examinations of the precipitates withextractive replicas in TEM reveal that morphology of cementite particles is complex they arecylinder-shaped particles, short bars particles and skeletal particles. OrientationsRelationships (OR) have been determined between ferrite and cementite particles with thinfoils in TEM by using Selected Area Diffraction, in a large number areas the Isaichev andBagaryatskii OR have been observed, a little Pitsch Petch OR have been obtained. The EBSDmap shows that the bainitic ferrite morphologies are both lath like and polygon. Themisorientations inside the laths are very small (0.5° misorientation point to point) andbetween laths we have the range 49-60° misorientation. The profile of misorientationsbetween point to point indicates higher frequency for the range 49-60°. Charpy test have beenperformed to analyze the effect of the microstructure on the fracture energy at lowtemperature. The results show that the fracture energy decreases when the content ofproeutectoid ferrite is high.

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.

Ce travail de thèse vise à comprendre et à modéliser les mécanismes de transformation de phase d'austénite en bainite dans un acier bas carbone faiblement allié et en particulier d'étudier une éventuelle sélection de variants à l'échelle locale. Une campagne d'essais de torsion a permis de déterminer des lois de comportement des phases austénitique et bainitique à haute température afin d'améliorer un modèle existant de sélection de variants et d'obtenir des échantillons présentant différentes microstructures de l'austénite avant transformation. Puis des essais de transformation bainitique isotherme ont été réalisés afin de pouvoir étudier l'influence de la température de transformation et de la taille de grain austénitique sur la sélection de variants. L'étude des microtextures a été réalisée grâce à la technique EBSD qui nous a permis de confirmer la présence d'une répartition non aléatoire des variants voisins au sein d'un ancien grain austénitique. Le modèle micromécanique de sélection de variants porte sur un critère d'auto-accommodation de la déformation de transformation entre certains variants afin de réduire l'énergie globale du système composé de l'austénite et de plusieurs variants de bainite. Les déformations de transformation sont déterminées à l'aide de la PTMC, puis les interactions entre les différents variants sont obtenues en résolvant un problème d'inclusions d'Eshelby. La confrontation des résultats expérimentaux avec ceux du modèle induit une meilleure compréhension des microstructures de martensite et de bainite formées à basse température par le critère d'auto-accommodation des déformations de transformation que celles formées à haute température.

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.

Seit vielen Jahren wächst die Nachfrage bezüglich sparsamer Fahrzeuge; die Autohersteller konkurrieren miteinander und werben mit neuen Fahrzeugkonzepten, in denen hochmoderne Werkstoffe ihre Anwendung finden. In dieser Arbeit werden Legierungskonzepte und entsprechende Warmwalztechnologien einer ultrahochfesten bainitischen Stahlsorte mit Restaustenit vorgestellt, die der genannten Anwendung angepasst werden können. Der gewählte Werkstoff gehört zu den Stählen mit mittleren Kohlenstoffgehalten, die sich nach der - im Rahmen dieser Arbeit entwickelten - Behandlung durch hohe Zugfestigkeit bei vergleichsweise hoher Bruchdehnung auszeichnen. Es werden erweiterte Modelle zur Beschreibung der Phasenumwandlung von Stählen im Bainitgebiet vorgestellt. Die Ergebnisse aus den Experimenten wurden genutzt, um die Modelle zu ergänzen und zu evaluieren. Dabei wird nicht nur der Warmwalzprozess, sondern auch die chemische Zusammensetzung der Stähle selbst optimiert. Die hier präsentierte Arbeit erstreckt sich über die gesamte Produktionskette und zeigt geeignete Herstellungsbedingungen, die in Betriebsanlagen leicht realisierbar sind und umgesetzt wurden.