Quenching and partitioning (Q&P) is a novel processing route that can create microstructures that combine high strength, ductility and easy formability without the excess use of alloying elements by also keeping the production cost low. Therefore, this method is ideal to be applied to steels that are used in the automotive industry. The material is initially annealed at a high temperature to form austenite. Then it is quenched to a temperature below the martensite start temperature to form a controlled fraction of martensite. Second annealing follows, at a lower temperature than the first one, that stabilises the austenitic phase at room temperature. It has been found that the austenite grain size at the end of the first annealing (prior austenite grain size) has a significant impact on the processing method, the final microstructure and consequently on the final properties of the material. In this thesis, phase-field modelling was employed to investigate the role of prior austenite grain size on the quenching and partitioning process. Simulations of the processing were performed for three different prior austenite grain sizes (6, 25 and 67 μm). Microstructures consisting of about 10.5% austenite and 89.5% martensite were created by simulating the first quenching of the Q&P process. Then, the evolution of the microstructure and carbon distribution were investigated for partitioning at 400 ◦C for times up to 4000 s, and quenching to room temperature. The results gave insights into the effect of the prior austenite grain size on the morphology of the microstructure and the kinetics of carbon during partitioning. It was found that a finer microstructure was derived with decreasing PAGS. Moreover, partitioning becomes more efficient because of the shorter distance that carbon has to diffuse, and the even spatial distribution of austenite. Finally, wide grain size distributions of the austenite that is present during partitioning can lead to significant variations in the carbon content among the austenite grains of the final microstructure, especially for short partitioning times. This affects the efficiency of the partitioning process and has an impact on the mechanical stability of the austenite in the final microstructure and consequently on the TRIP phenomenon.

Abstract The Quenching and Partitioning (Q&P) process starts with an austenitization process followed by rapid cooling to a temperature named quenching temperature ranged between the martensite start (Ms) and martensite finish (Mf) temperatures to establish initial fractions of martensite and austenite. Afterwards, the material is reheated to a particular partitioning temperature and for a given time to allow the carbon diffusion from martensite to austenite. Finally, the steel is quenched to room temperature and a fraction of fresh untempered martensite may form from the least stable austenite. The whole process determines the phase fraction of the constituent phases, namely: tempered martensite (M1), retained austenite, and un-tempered martensite (M2). This combination of phases has the potential to simultaneously improve strength and elongation of steels due to their small grain sizes and the transformation-induced plasticity resulting of the transformation of the retained austenite into martensite during a deformation.  It is known that the fraction of retained austenite of Q&P steels depends on the quenching temperature, showing a bell-like relation. The volume fraction of retained austenite shows a maximum at a particular quenching temperature somewhat intermediate between the Ms and Mf temperatures, known as the optimum quenching temperature, below and upper which the volume fraction of retained austenite decreases in either way.  Recently, it is reported [1] that retained austenite fraction after reheating up to 700oC followed by quench losses its dependency of quenching temperature and reaches a constant value. This can be a manner to further control phase fractions and consequently the mechanical properties of Q&P steels. The mechanisms by which the volume fraction of austenite changes to reach that constant value are not well understood and are the focus of the present study.  In the present work, Q&P samples with chemical composition 0.31C-4.58Mn-1.52Si (wt.%) were reheated up to 700oC and quenched directly to compare the retained austenite fractions of reheated and un-reheated (Q&P) samples. It was found that the retained austenite fractions tend to have a constant value after reheating to 700oC followed by quenching, whereas the retained austenite fraction of the Q&P samples showed an optimum value and usual dependency with quenching temperature. It was also observed that the highest and lowest drop of retained austenite occurred in the reheated Q&P samples at 160oC and 80oC of quenching temperature. These samples were reheated to 900oC to have a complete observation of microstructural events by means of dilatometry, optical and electron scanning microscopy, hardness, and X-Ray diffraction measurement.  Interrupted reheating was applied to understand the microstructural changes during reheating. The two samples were interruptedly reheated and directly quenched at 450oC, 530oC, 610oC, 720oC, and 740oC to measure the retained austenite fraction and to capture the corresponding microstructure around the observed changes in slope. The retained austenite fractions evolution of both samples were measured and compared. It was found that the decreasing of retained austenite fraction started around the critical temperature of 600oC and remained up to 700oC of reheating temperature. Further reheating process increased the retained austenite fraction.  In the specimen showing the lowest drop of retained austenite fraction, retained austenite decomposed by two mechanisms. The first one was the formation of discrete particles of cementite, which originally came from film type austenite. The second was the formation of globular carbides within tempered martensite. On the other hand, river-like patterns were observed surrounding the martensite region. The river-like patterns mainly consist of martensite from new austenite that formed between 720oC and 750oC. In the highest drop of retained austenite fraction sample, retained austenite decomposed by three mechanisms. The first two mechanisms were similar to the previous sample. The last mechanism was the formation of substructure within Martensite-Austenite islands. River-like patterns were also observed surrounding the martensite region, consisting of martensite which originally came from new austenite as a product of (martensite and) carbide transformation that formed between 720oC and 745C and lath martensite which originally came from the unstable austenite forming upon reheating to 740oC.