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.

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.

L’alliage Ti-5553 destiné à la fabrication des trains d’atterrissage suit au cours de sa mise en forme, un schéma thermomécanique assez complexe composé par des étapes de forgeage successives dans les domaines monophasé β et biphasé α+β. Ainsi dans le but de rendre possible l’optimisation de ses gammes de forgeage, un important développement des connaissances sur la rhéologie et sur les évolutions microstructurales au cours du traitement thermomécanique est donc nécessaire pour l’entreprise Messier Bugatti Dowty. L’objectif ici est de déterminer expérimentalement la rhéologie et de modéliser le comportement mécanique ainsi que la caractérisation des microstructures de l’alliage Ti-5553 au cours des séquences de déformations dans les domaines et. La détermination expérimentale de la rhéologie à chaud de l’alliage Ti-5553 a été réalisée d’une part au moyen d’essais de compression uniaxiale dans les domaines biphasé et monophasé, ce qui a permis d’identifier une loi de comportement du matériau dans le domaine pour les deux états "billette" et "pièce forgée", de décrire le comportement rhéologique du Ti-5553 dans les domaines de température, de vitesse et de déformation comprises respectivement entre 720 et 990 °C ; 0,001 et 1 s–1 ; 0,1 et 1,2. C’est ainsi qu’un modèle rhéologique a été proposé, basé sur la Loi de Hansel et Spittle qui prend en compte l’évolution de la contrainte d’écoulement du matériau en fonction de la vitesse de déformation et de la température.Par ailleurs, les analyses microstructurales réalisées en microscopie optique, aux rayon X, au MEB et en EBSD ont permis de caractériser les tailles des grains  et , des textures des états initiaux et de déformés, et de mettre en évidence un phénomène de précipitation dynamique de la phase . / Ti-5553 alloy used for landing gear manufacturing has a complex thermomechanical diagram during hot working process which consists of successive forging steps in the single phase β and the two phase α+β regions. For this purpose, in order to optimize theTi-5553 forging process in Messier-Bugatti-Dowty Company, significant development of knowledge of rheology and the microstructural evolution during thermomechanical processing is necessary. The aims of this work are: i) to find out experimentally the rheology, ii) to model the mechanical behavior, and iii) to characterize the microstructural changes during different strain sequences in theα+β and β regions.Uniaxial compression tests were carried out in order to determine the rheology of the Ti-5553 alloy in the single and the two-phase region. The latter provide the behavior of the alloy in the two initial states, “billet” and “as forget”. This information was used to determine the rheological behavior of the material in the temperature range 720 to 990 ˚C, strain rate range 0.001 to 1 s-1 and strains in the range 0.1 to 1.2. A rheological model of the material behavior based on Hansel-Spittle equations was proposed which takes into account the dependence of the flow behavior of the material with strain rate and temperature.Moreover, characterisation methods such as optical, scanning electron microscopies, X-rays and EBSD analyses were used to examine the microstructures in the initial state (undeformed) and the deformed material. These techniques allowed the measurement of alpha and beta grain sizes as well as the texture of the material at different conditions (undeformed and deformed material). The results also indicated that a dynamic α-phase precipitation phenomenon in the material can take place during the hot working process.

Thermomechanical processes such as hot rolling have been used in the industry for a long time to process and shape metals to a desired form with specific properties. However it can be difficult to make changes to the different process parameters. That's where it is beneficial to use a hot deformation simulator such as the Gleeble 3800-GTC. It can be used to test metals in a controlled environment where the deformation, temperature and other parameters are easily changed. When the machine uses a Hydrawedge module, it is able to simulate hot rolling using uniaxial compression at high temperatures. Swerim AB has one such machine and has requested to investigate what occurs inside a specimen during testing in the Gleeble, specifically inside two low-alloyed steels with a hardness between 400 and 500 HV. Such tests were replicated using LS-DYNA, an FE software. The goal was to acquire true stress-strain graphs that showed similar behaviour to the data from the Gleeble and plots of the effective plastic strain which could be correlated to the grain structure pattern inside the deformed cylinders. An FE-model was created which replicates the procedure. The model was verified through numerous steps. An initial mesh verification was done where the simulation time took at least 5 hours and at most 86 hours. Using a technique called mass scaling, the elements inside the model were manipulated using additional mass to increase their time step and reduce the computational time. A verification of the mass scaling was done where the computational time was weighed off against accuracy. Afterwards the friction had to be verified where it was found that the Gleeble test specimens were deformed more than necessary which was taken into account and the models were adjusted for friction verification. After all was said and done, the model had a reasonable friction coefficient with an optimal mesh and mass scaling configuration. The resulting model simulated a test of 0.5 seconds in 15 minutes and only costing at most 10 MPa in accuracy when experimental results have maximum values between 110 to 220 MPa depending on the scenario. This equals an approximate error of around 5-10%. When investigating the grain structure after 100 seconds of relaxation, the computational time amounted to 52 hours but could be reduced to 12 hours when simulating 30 seconds as there was no change in the effective plastic strain after that time. The final model has a high enough accuracy which, when combined with the Gleeble, is able to confirm material models and describe what occurs in the material during conditions akin to hot rolling.