Tailoring bainitic transformation and enhancing mechanical properties of carbide-free bainitic steel via high-temperature ausforming

Abstract

Ausforming, as a classical thermomechanical treatment process, provides a promising solution for evading the strength-ductility trade-off in carbide-free bainitic (CFB) steel. However, systematic investigations of the relationships among ausforming strain, bainitic transformation kinetics, bainitic morphology and RA features, together with their effects on mechanical properties were scarce in the literature. In the present work, high-temperature ausforming (HT-AF) at 650 °C with different strains was employed in a CFB steel. With the increase of deformation, the prior austenite grain (PAG) gradually flattens. The content of retained austenite (RA) at room temperature increases and RA morphology changes from “film and a small amount of block” to “block and a small amount of film”. Systematic high-resolution dilatometry and transmission electron microscopy (TEM) characterization were performed to reveal the effect of dislocation substructures on bainitic transformation kinetics. The results show that the microband structures reduce the transformed volume fraction and the maximum transformation rate. But the small deformation (10–25%) promotes the initial transformation rate, when the deformation increases to 35–45%, the bainite transformation kinetics is inhibited due to the high mechanical stability of deformed austenite. Given these results and the variation of bainitic morphology, the mechanism of microstructure evolution, especially the RA evolution is elucidated. Compared with the sample without ausforming, HT-AF improves the strength and ductility simultaneously, achieving outstanding mechanical properties (an ultimate tensile strength of 1578 MPa and uniform elongation of 11.1% in the AF45 + AT sample). This is attributed to a large amount of blocky RA providing the dislocation absorption and sustained TRIP effect, the former promotes the ductility and the latter effectively enhances the work hardening capacity.