Effect of Prior Austenite Grain Size on Bainite Sheaf in Silicon Steel

Adding about 2wt% silicon to the carbon steel can produce carbide free bainite under the proper heat treatment. This kind of structure is different from normal bainite which consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. Therefore, carbide free bainite has more toughness and strength compared with the normal bainite. However, the development of this microstructure in silicon steel is still not well understood and need further study.
The present study attempts to control the austenitizing temperature to explore the microstructure development with the different transformation temperature and the effect of prior austenite grain size on bainite sheaf via dilatometer in different carbon concentration silicon steel. Because bainite transformation rate is nucleation control. The nucleation sites are mainly at the prior austenite grain boundary so that the finer prior austenite grain will increase the transformation rate due to the much more grain boundary area; Conversely, the greater prior austenite grain will decrease the transformation rate. The SEM, TEM and EBSD results show that different prior austenite grain size will influence the morphology, sheaf length and sheaves variant of bainite. With the increase of the prior austenite grain size, the sheaf length will longer, the sheaf variant will decrease and the shape of bainite sheaf become smoother. Besides, changing the transformation temperature or carbon concentration influences the bainite sub-unit width. The sub-unit width is increased when the transformation temperature increased or carbon concentration decreased. From the EBSD analysis, the texture of bainite structure have been studied. The result shows the variant selection of bainite sub-unit during growth.