Transformation phenomena during quenching & partitioning heat treatment

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Quenching & partitioning (Q& P) steel is a promising candidate for automotive applications because an excellent balance of high tensile strength and good elongation can be obtained with low alloy compositions. Transformation phenomena during Q& P process are, however, not yet fully understood due to their complexity. Carbon partitioning in this process has been suggested to be controlled by the constrained carbon equilibrium (CCE) model, which assumes that competing reactions such as bainite transformation and carbide formation are completely suppressed. However, such competing reactions have repeatedly been observed in the Q& P process, and hence the microstructural changes are in reality much more complicated than the ideal CCE situation. This thesis aims to reveal the complicated competing phenomena that occur during the Q& P heat treatment by separating each of them including carbon partitioning from martensite to austenite, bainite transformation, and carbide precipitation inside the martensite. This is essential to enable more precise prediction of the microstructures and resultant mechanical properties of advanced high-strength steels produced via the Q& P process.