Martensitic Transformation Crystallography And Finite Element Simulation Of Microstructure In Steels

The martensitic transformation is a diffusionless and displacive transformation, and the surface relief caused by shear, as one of major features of martensitic transformation,corresponds to unique shear angle for a given material. Therefore, the previous research results are summarized in the thesis to verify the shear mechanism of martensitic transformation. In these research,atomic force microscopy was at first used to accurately measure the surface relief angle,and then the shear angle was calculated based on the surface relief angle,finally the calculated shear angle was compared with the theoretical shear angle predicted by phenomenological theory of martensite crystallography (PTMC). These research results effectively deny the viewpoint on the non-shear mechanism of martensitic transformation.Recently, electron backscattering diffraction (EBSD) is rapidly developed and begins to be used in the research of martensitic transformation crystallography. A novel heat treatment process called quenching-partitioning-tempering (Q-P-T) was proposed by Xu Zuyao on the base of Q&P process developed by Speer et al. As the carbide precipitation is not permitted based on the theory of Q&P process, which excludes the potential of precipitation strengthening. Xu proposed a Q-P-T process as a modi?ed Q&P process to excavate the effect of precipitation strengthening by the addition of Nb, V and Ti elements. Since the precise measurement of orientation relationship between martensite and retained austenite in the Q-P-T steels has not yet been performed, EBSD is used to analyze microstructure and crystallographic characteristics in Q-P-T steel. Miyamoto et al. developed a method to quantitatively determine the orientation of austenite in the condition without retained austenite in low carbon steels. In this thesis, the precision of this method is first confirmed by EBSD measurement for low and medium carbon Q-P-T steels in the condition with considerable retained austenite, and then a new method is developed to quantitatively determine the orientation relationship between martensite and retained austenite.The study of strength and ductility mechanism plays an important part in development of advanced high strength steels. Finite element analysis (FEA) is a mathematical approach, which can be used to simulate actual system. By introducing the effect of stress relaxation from martensitic transformation in previous FEA model, a modified model is established in the thesis and is used to simulate the microstructural evolution in Q-P-T steel. By comparing simulation and previous experiments, it indicates that simulation result based on modified model is better consistent with experimental observation than that based on the previous model. The modified simulation result plays a role in revealling the mechanism of ductility enhancement by strain-induced martensitic transformation.