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Low carbon bainitic steels microalloyed with Nb, Ti and V are widely used for the pipeline, construction and automobile industries because of their excellent combination of strength, toughness and weldability. Boron as another major alloying element has been also frequently used in this type of steels since the 1970s. The purpose of adding boron is to improve the hardenability of the steel by promoting bainite formation. / It has been realized that Boron can only be effective as a strengthening element when it is prevented from forming BN and/or Fe23(C, B) 6 precipitates. Therefore, Boron is always added together with other alloying elements which are stronger Nitride or Carbide formers, such as Ti and Nb. However, the formation of complex bainitic structures and the interaction with precipitates at industrial coiling temperature are not adequately understood. / In this study, the effect of boron on the microstructure and mechanical properties of a low carbon Nb-B steel was studied by a hot compression test (50% reduction at 850°C) followed by quenching samples into a salt bath. The microstructures of the tested samples were examined through optical microscopy and SEM; and the mechanical properties of these samples were investigated by micro-hardness and shear punch tests. / The results indicate that during thermo-mechanical controlled rolling (TCR), the final properties of the products not only depend on the applied deformation but also depend on the coiling temperature where phase transformation takes place. According to the investigation, two strengthening mechanisms are responsible for the strength of the steel at the coiling temperature: phase transformation and precipitation. Under optical microscopy, the microstructures of all specimens appear to be bainite in a temperature range from 350°C to 600°C without distinct differences. However, the SEM micrographs revealed that the microstructures at 550°C are very different from the microstructures transformed at the other holding temperatures. / Two strength peaks were observed at 350°C and 550°C in the temperature range studied. It is believed that the NbC precipitates are the main contributor to the peak strength observed at 550°C because the kinetics of NbC is quite rapid at this temperature. The strength peak at 350°C is mainly due to the harder bainitic phase, which formed at relatively lower temperature.

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<p> The fracture behaviour of a medium strength bainitic steel (SAE 4340 in the 11 as transformed and in the "warm rolled" condition) . and four carbon-manganese structural steels (in the hot rolled ferritepearlite condition) was investigated. The purpose was to isolate those features of the microstructure which exert control over the fracture properties. </p> <p> The detailed nature of the microstructure of the steels was studied with transmission and scanning electron microscopy, qualitative x-ray analysis and quantitative metallography. An attempt was made to correlate the fracture behaviour with the microstructure through models which relate to the structure properties to the unnotched tensile properties. </p> <p> In the case of the bainitic steels it was found that the carbide morphology, dislocation substructure and prior austentite grain size have the major influence on fracture properties. In contrast, the fracture properties of the structural steels were controlled by the volume fraction of inclusions and to some extent by the shape of the inclusions. </p> / Thesis / Master of Engineering (MEngr)

Carbide-free bainitic (CFB) steels have gained increasing attention in recent years because of their excellent mechanical properties. The excellent combination of strength, ductility and toughness achieved in these steels is only matched by that of Maraging steels which cost 10 to 100 more than the carbide-free bainitic steels. The excellent mechanical behavior of CFB steel is mainly due its complex microstructure (bainitic ferrite, retained austenite and martensite) consisting of a high strength phase (ultra fine bainitic ferrite) and TRIP effect from retained austenite. Carbide formation is avoided due to high silicon content which suppresses cementite precipitation from austenite. The effect of bainitic transformation time on the microstructure and mechanical properties was investigated in a steel containing 0.4%C-2.8%Mn-1.8%Si. The microstructure was characterized using optical and transmission electron microscopy; it consisted of bainitic ferrite, martensite and retained austenite. This microstructure exhibited an extended elasto-plastic transition leading to very high initial work hardening rates. The work-hardening behavior was investigated in detail using strain-path reversals to measure the back-stresses. These measurements point to a kinematic hardening due to the mechanical contrast between the microstructural constituents. The strain aging effect at room temperature on the CFB steel was also been analyzed in great detail. The static strain aging effect at room temperature can not be overlooked in the carbide free bainitic steel. After isothermal bainite heat treatment, the yield strength of the material is increased by about 80MPa, and the ultimate tensile strength is improved by more than 100MPa after aging at room temperature for one week. This phenomenum could be related to the interactions between carbon atoms and the dislocations, grain boundaries and the redisual stresses. Examination of the fracture surfaces indicated that the prior austenite grain boundaries play an important role in the fracture process. A set of experiments were designed to study the effect of ausforming on the microstructure and mechanical properties of CFB steels. Based on its mechanical behavior under tensile tests and microstructural analysis by EBSD, the TRIP effect was contributing to the work hardening behavior. The changes in morphology and variant selection of the bainitic ferrite lath in the ausformed carbide free bainitic steel were also observed. A new set of chemistry was design with reduced carbon and manganese content to further improve the weldability and the reproducibility of the carbide free bainitic steel. / Thesis / Doctor of Philosophy (PhD)

L’objet de cette thèse est la prise en compte des effets du fraisage de finition dans un modèle d’endommagement en fatigue. Les procédés d’usinage tels que le fraisage sont connus pour imposer de fortes sollicitations thermomécaniques, pouvant altérer les propriétés géométriques (rugosité, arrachements) et mécaniques (contraintes résiduelles, écrouissages) en surface et sous-surface des pièces produites. Ces propriétés, regroupées sous le terme d’intégrité de surface, sont susceptibles d’affecter significativement la tenue en fatigue des pièces fabriquées. Cette problématique a été traitée pour le cas particulier du fraisage de finition à l’outil hémisphérique d’aciers à microstructure bainitique. Plusieurs axes d’étude ont été suivis, à commencer par la mesure et la caractérisation de l’intégrité de surface, et en particulier de son caractère anisotrope. Dans un deuxième temps, le rôle joué par l’intégrité de surface lors de sollicitations en fatigue a été mis en évidence au travers d’une campagne d’essais de flexion portant sur différents types de surface. Ces travaux à dominante empirique ont été complétés par l’étude et l’amélioration d’une approche hybride visant à mêler données expérimentales et modélisation pour prévoir rapidement et efficacement les profils de contraintes résiduelles induits par le procédé. Enfin, la dernière partie de l’étude a porté sur la prise en compte des résultats ainsi obtenus dans un modèle d’endommagement dit à deux échelles, dans le but de représenter, et dans la mesure du possible de prévoir, l’effet de l’intégrité de surface sur le comportement en fatigue du matériau. / This thesis focuses on the effects of finishing milling on fatigue damage. Machining processes such as milling are known to incur high thermomechanical loadings, which alter the geometrical (roughness) and mechanical (residual stresses, strain hardening) properties of the surface and sub-surface of produced parts. These properties, designated as surface integrity, are likely to affect significantly the fatigue strength of machined parts. The problem has been studied here in the case of the ball-end tool finishing milling of bainitic steels. Several approaches were followed, starting with the measurement and characterization of surface integrity, and especially of its anisotropic nature. Secondly, the role played by surface integrity during fatigue behaviour was highlighted through a bending test campaign including different surface types. These mainly empirical works were completed with the study and improvement of a hybrid approach aiming at combining experimental data and modelling in order to predict quickly and efficiently the residual stress profiles induced by the process. Finally, the last part of the study has dealt with taking account of the results thus obtained in a so-called two-scale damage model, in order to describe, and insofar as possible, to predict the fatigue behaviour of the machined material.

The need to develop light-weight and high strength materials for car frames which improve fuel efficiency and provide increased passenger safety during dynamic events such as automobile crashes has been the focus of the steel and automobile industries for the past 30 years. In recent years, the development of high strength steels such as multi-phase TRIP (Transformation-Induced Plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility. The savings in automobile weight is provided by the inherent strength of TRIP steels which allows for the use of thinner sections. The TRIP effect is characterized by the phenomenon known as strain-induced martensitic transformation (SIMT) which enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. This results in a resistance to local necking which subsequently enhances the strength, ductility, and formability of such steels. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in type 304 austenitic and multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to, the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels, and the volume fraction and morphology of the retained austenite islands. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.

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