Study of the mechanisms of carbon enrichment in austenite in Q&P steels

L’allègement dans le secteur de l’automobile revête un enjeu important du fait de normes d’émission de CO2 de plus en plus drastiques, de la nécessité de réduire la consommation en carburant des véhicules et d’une aspiration sociétale à une économie « plus verte ». Pour répondre à ces défis et dans un souci de sécurité et de contrôle des coûts, l’industrie automobile étudie actuellement la possibilité de développer et de produire une 3ème génération d'aciers à très haute résistance. Ils résultent de traitements thermomécaniques généralement innovants, possèdent des microstructures complexes et des propriétés mécaniques améliorées. Le procédé de Quenching and Partitioning (Q&P) est le traitement le plus prometteur, il consiste en une trempe sous la température de début de transformation martensitique Ms, puis, d'un réchauffage et d'un maintien au-dessus de la température initiale de trempe (QT). L'étape de maintien est appelée "étape de partition", car un enrichissement en carbone de l'austénite est attendu. Les propriétés mécaniques exceptionnelles des aciers Q&P sont dues à leur microstructure duplexe complexe : de très fins îlots d'austénite résiduelle imbriqués dans une matrice martensitique revenue et/ou fraîche. Bien que les mécanismes d'enrichissement en carbone de l'austénite résiduelle lors de l'étape de partition soient encore débattus dans la littérature, il existe des preuves tangibles qui attestent d’un phénomène de partition du carbone de la martensite vers l’austénite. Cependant, la formation de bainite et de carbures dans la martensite soulève la question de l’influence de ces réactions et de leurs interactions sur les mécanismes et les cinétiques d’enrichissement en carbone de l'austénite. Il s'agit clairement d'un sujet d'intérêt puisque les propriétés mécaniques de ces aciers reposent principalement sur la teneur en carbone des îlots d’austénite.Cette thèse qui repose sur une approche expérimentale multi-échelle couplée à une approche théorique en champ moyen, a pour ambition d’apporter des éléments de réponse aux mécanismes d’enrichissement en carbone de l’austénite dans un aciers duplex Q&P à très haute résistance de composition Fe-0,3 C-2,5Mn-1,5Si...The need to reduce the fuel consumption of vehicles while increasing safety led the automotive industry to develop a 3rd generation of Advanced High Strength Steels. Such steels combine innovative processing routes, complex microstructures, improved mechanical properties and are a possible response in vehicle lightweighting. The Quenching and Partitioning (Q&P) process is the most promising route and involves quenching below the martensite start temperature followed by a reheatingand ageing above the initial quench temperature (QT). The ageing step is termed “partitioning step” since carbon enrichment in austenite is expected to occur during this stage. The exceptional mechanical properties of Q&P steels come from their complex duplex microstructure: very fine austenite island retained at room temperature embedded in both recovered and fresh martensite. Although the mechanisms of carbon enrichment in retained austenite during the partitioning step are still debated,strong evidences of carbon partitioning from martensite to austenite exist. However, both the formation of bainite and carbides into martensite raise the question of the effects of competitive reactions on the carbon enrichment in austenite. It is clearly a topic of interest since the benefits of such a treatment in terms of improved mechanical properties depends strongly on the austenite stability and thus on the level of carbon enrichment in austenite during the partitioning step.This thesis aims at combining an innovative multiscale experimental methodology with an original theoretical approach providing a unique opportunity to give some clarifications regarding the microstructure evolution and the mechanisms of carbon enrichment into austenite. After having determined the optimum Q&P parameters using dilatometric and XRD measurements, a Q&P treatment at three different QT (260, 230 and 200°C) and at a partitioning temperature of 400°C was applied to a model Fe-0.3 C-2.5Mn-1.5Si steel. The dilatometric data combined with an SEM image analysis study showed that bainite forms during the partitioning step. The presence of bainite was also confirmed by in-situ High Energy X-Ray Diffraction. While bainite was shown to appear as carbide free laths, tempered martensite showed an advanced state of intra-lath precipitation. The combination of atom probe tomography (APT) and TEM technics showed that theses carbides are transitional andboth η and ε carbides were observed. Their carbon content ranged from 20.0 to 27.7 at.%. APT measurements also highlighted carbon segregation on martensite defects during the initial quench and calculation of the evolution of the carbon excess concentration on laths boundaries suggest that desegregation occurs along the Q&P treatment. In-situ HEXRD permitted to follow the austenite lattice parameter evolution and it was shown that austenite is subjected to a sequence of tensile andcompression state induced by the formation of martensite. A model for the coefficient of thermal expansion of austenite taking into account its stress state was successfully developed. The evolutions of carbon content into austenite for the three QT were determined. Surprisingly the carbon enrichment into austenite was shown not to depend on QT. It was also shown that the increase of carbon content in austenite results from both carbon partitioning and bainite contributions. Lastly, an originaltheoretical approach was developed. It was evidenced that bainite continues to form while partitioning process is rapidly completed, thus bainite transformation controls the maximum austenite carbon enrichment at 400°C, independently of QT. The contribution of partitioning from martensite was shown to be larger with decreasing QT. The developed model successfully described the experimentally observed phase transformations and austenite carbon enrichment by taking into account theinteractions between carbon partitioning, bainite transformation and carbide precipitation