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The quenching and partitioning (Q&P) process is a new heat treatment for the development of advanced high strength steels. This treatment consists of an initial partial or full austenitization, followed by a quench to form a controlled amount of martensite and an annealing step to partition carbon atoms from the martensite to the austenite. In this work, the microstructural evolution during annealing of martensite-austenite grain assemblies has been analyzed by means of a modeling approach that considers the influence of martensite austenite interface migration on the kinetics of carbon partitioning. Carbide precipitation is in the and three different assumptions about interface are considered, ranging from a completely interface to the mobility an incoherent ferrite-austenite interface. Simulations indicate that different interface mobilities lead to profound differences in the evolution of microstructure that is predicted during annealing.

Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

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The application of the quenching and partitioning (Q&P) process in steels involves a microstructural evolution that is more complex than just the formation of martensite followed by carbon partitioning from martensite to austenite. Examples of this complexity are the formation of epitaxial ferrite during the first quenching step and the formation of bainite, carbides, and carbon gradients as well as migration of martensite/austenite interfaces during the partitioning step. In this work, recent investigations on the mechanisms controlling microstructural changes during the application of the Q&P process are evaluated, leading to phase-formation based concepts for the design of Q&P steels. NEW strategies for the creation of advanced high strength steels with improved mechanical properties of strength, toughness, and ductility are based on the development of microstructures consisting of ultrafine phases formed in nonequilibrium conditions such as martensite and bainite in combination with retained austenite. [1] The refined and highly dislocated martensite and bainite contribute to a simultaneous increase of strength and toughness. Retained austenite contributes to the improvement of the strength/ductility combination via the transformation induced plasticity (TRIP) effect and to the improvement of the toughness if the retained-austenite grains have a filmlike morphology. One of the most innovative procedures to create microstructures consisting of martensite and retained austenite is the so-called quenching and partitioning (Q&P) process. [2] This process starts with a total or partial austenitization, followed by a quench of the microstructure to a temperature (quenching temperature) below the martensite start (M s ) temperature but above the martensite finish (M f ) temperature to form a controlled fraction of martensite. This microstructure is then subjected to a treatment at the same or higher temperature (partitioning temperature) in order to accomplish the carbon diffusion from the carbon-supersaturated martensite to the neighboring austenite. Finally, the material is quenched to room temperature and the austenite that has been sufficiently carbon enriched remains metastable at room temperature, whereas the rest transforms into martensite.Investigations on the Q&P process were mainly focused on the application of this heat treatment to steels with chemistries very close to commercial TRIP steels, [3][4][5][6][7][8] designed to promote bainite formation, whereas studies on steels specially designed to be subjected to the Q&P process are fewer. [9][10][11] Depending on the steel composition and the particular heat treatments, formation of bainite, ferrite, and carbides during the Q&P process can overlap with carbon partitioning from martensite to austenite, reducing the effectiveness of this heat treatment leading to the desired microstructures. An adequate theoretical knowledge of the mechanisms occurring during the Q&P process would lead to a well control of these overlapping phenomena. In the present work, ...