The Complex Phase Transformation of Austenite in High Strength Linepipe Steels and Its Influence on the Mechanical Properties

During processing of low carbon high strength linepipe steels, complex microstructures are usually obtained. Toughness of the steels is found to be strongly dependent on the complex microstructures. Since the microstructural and chemical condition of austenite is very important for the subsequent microstructures, austenite grain coarsening and recrystallization temperatures were determined. The results showed addition of 0.3wt% more chromium can reduce about 100°C of the grain coarsening temperature. Thus, the alloy design should be considered together with thermomechanical processing to avoid the mixture of austenite grain size. It was found that Bs temperatures of steel have a wide range from 400°C to 580°C, depending on cooling rates. The formation of martensite-austenite (MA) constituents and bainitic transformation were investigated in isothermal treatment and continuous cooling conditions. The carbon diffusion was discussed from the view point of thermodynamics and kinetics to explain the formation of MA during bainitic transformation. It was found that controlling carbon diffusion is most important point for the formation of MA. Some experiments were designed and the results confirmed the thermodynamics analysis. In addition, the crystallographic orientations of bainite formed at different bainite transformation temperatures were also determined by EBSD analysis. The orientations of bainite are irrational, but two typical orientations were found. The orientation is near <113> at a higher transformation temperature and the orientation is near <331> at a lower transformation temperature. The crystallographic packet size of bainite is large when the orientation is near <113>. Coincident Site Lattice (CSL) grain boundaries were introduced to explain its relationship to toughness. As proposed in this thesis, the size and volume fraction of MA, crystallographic packet size and CSL grain boundaries are the three predominant factors affecting the impact toughness of steels. Thus, some methods were proposed for impact toughness improvement. In this regard, a schematic CCT diagram was developed based on the classification of bainite and the distribution of MA within each classification. These results could provide some guidance for improved understanding of the complex microstructures of these steels