Effects of ausforming temperature on carbide-free bainite transformation and its correlation to the transformation plasticity strain in a medium C- Si-rich steel

Abstract

Ausforming treatments at different temperatures were applied to a medium CSi rich carbide free bainitic steel, and the role of transformation plasticity on the evolution of the microstructure is discussed. Three ausforming temperatures were selected prior to isothermal bainite transformation at 325 °C. Deformation was applied above the bainite start temperature, BS, (600 °C), below BS (400 °C), and at the isothermal transformation temperature (325 °C). A combination of high-resolution dilatometry, XRD, SEM, and EBSD was used to investigate phase changes, microstructure evolution, and variant selections for the different ausforming conditions. The results indicated that the transformation rate was enhanced for all deformation conditions compared with the pure isothermal condition. Ausforming above BS did not modify the morphology of the carbide free microstructure, while ausforming below BS led to an asymmetrical morphology in the specimen. The alignment of the bainite plates can be at a random or a specific angle, depending on the supercooled austenite condition prior to the bainitic transformation. The thickness of the bainitic ferrite plates was refined to about 100 nm for the ausforming cycle at 325 °C resulting in a hardness of values of 540 HV. Transformation plasticity strains increased with increasing bainite transformation and were more intense with increasing microstructure alignment at the two lower ausforming temperatures. Texture studies revealed that both pure isothermal and ausforming at temperatures above BS resulted in an almost random texture, while a strong texture was obtained when ausforming below BS, a which was attributed to variant selection.