Unforeseen influence of the prior austenite grain size on the mechanical properties of a carbide-free bainitic steel

Abstract

The reasons behind the limited impact toughness of medium manganese, medium carbon carbide-free bainitic steels has been discussed in this manuscript. Various hot-rolling schedules have been employed to obtain microstructures with three different prior austenite grain sizes (PAGSs): coarse (CG = 45 ± 5 μm), medium (MG = 18 ± 4 μm) and fine (FG = 8 ± 2 μm) grain sizes. Although the coarse and middle grain size microstructures present similar levels of strength (tensile strength ∼1000 MPa) and ductility (36% and 41%, respectively), the impact toughness measured at room temperature improves significantly from 17 J to 83 J in the middle grain size one because of the lower ductile-to-brittle transition temperature (DBTT) temperature. Compared to the other microstructures, the fine grain size one contains higher contents of blocky unstable retained austenite, which are responsible for the enhanced strain hardening behavior under tensile testing but can be harmful for the toughness. Besides, during the first stages of straining these blocky austenite islands transform effortlessly, deteriorating the ductility (30%) and toughness (58 J) in spite of keeping a similar DBTT as the microstructure with the medium grain size.