No abstract

No abstract

The effects of cooling rate and austenite structure on bainite formation was investigated by means of electron backscatter diffraction analysis and processing of obtained orientation data. Variant pairing tendency of bainitic ferrite was found to depend on the austenite grain size, austenite plastic deformation and cooling rate. In the bainite formed at low cooling rate the variant pairs having the same Bain axis correspondence are more frequent, while at high cooling rate the variant pairs having the same parallel correspondence of close-packed planes are formed side by side preferably. At the same time, these features are influenced significantly by structural state of parent austenite.

No abstract

The method of quantitative metallography has been developed recently making it possible to evalutate bainitic microstructure of modern pipeline steels 1. When using this method, the microstructural inhomogeneities identified as the blocks of bainite with lath-like morphology was revealed among different morphological forms elongated along the rolling direction. Moreover, these blocks were shown to reduce the essential mechanical properties of the plate. In the present study, the crystallographic features of such inhomogeneities was examined by means of the SEM using EBSD analysis in order to characterize them more rigorously and clarify their nature.Industrially processed low-carbon microalloyed steel was investigated. Steel plate was hot rolled from a 300-mm slab to a 27.7-mm plate under industrial conditions. Last passes were conducted at nonrecrystallization temperatures 750±40ºC, and a true strain accumulated therewith was about 1.6. After rolling, the steel was cooled with a rate about 10 K/s. The samples for SEM examination were prepared from a central region of the steel plate. EBSD analysis was carried out by SEM Quanta 200 3D FEG using the EDAX Pegasus system with a step size 0.5 m. The orientation relationship (OR) between  and  phases, the statistics of crystal misorientations and the orientations of the former austenite were determined for comprehensive characterization of the microstructure. Fig. 1a presents an example of the microstructure, where the coarse-grained region corresponds to the inhomogeneity found previously by optical metallography. The differentiation of fine-grained and coarse-grained regions is supported in Fig. 1b by grain average misorientation (GAM) mapping. The scale of color differentiation used here was suggested recently to distinguish various -phase species 2. One can see that the fine-grained regions are colored partially by blue, green and yellow. These colors indicate a relatively low level of crystal curvature. In particular, the blue color corresponds to the polygonal ferrite 2. Alternatively, coarse-grained regions are colored in red (high level of crystal curvature). This corresponds to species formed by a displacive transformation and thus confirms that we are dealing with bainite.