Bimodal Microalloyed Steel Structure under Quasi-static and Dynamic Loading

The ultrafine-grained (UFG) and bimodal structures produced by an advanced thermomechanical processing, namely rolling, in microalloyed steel were subjected to microstructural and plastometric analyses under conditions of quasi-static and dynamic loading. Based on dilatometric studies, thermomechanical processes for laboratory rolling were chosen. This allowed, at a constant total strain value, to obtain microstructures with different compositions and morphologies of individual components. The microstructure development was examined to determine how the thermomechanical parameters and microalloying elements promote refinement and compositions of the final microstructure. Applied schedules of the thermomechanical processing allow the production of Ultrafine-Grained (UFG) microstructures with high inhomogeneity in the final products. Several characteristic samples were selected as a particularly complex and unexpected representation of the obtained microstructures for further research. Plastometric tests were conducted, encompassing compression and tension tests under quasi-static loading with digital image correlation (DIC) analysis. Additionally, dynamic loading was applied using a drop hammer and a Split Hopkinson Pressure Bar (SHPB) at strain rates ranging from 800 to 2000 s-1. Samples deformed under such conditions were subjected to microstructural analysis and mechanical properties measurements. It was observed that the use of different combinations of TMP parameters can result in the formation of specific microstructures, which, in turn, are the source of an attractive mechanical response under dynamic loading conditions. This opens up new possible areas of application for such common structural materials as microalloyed steels. The substantial complexity of the phenomena, arising from the microstructure's response to dynamic loadings, poses formidable challenges for the theoretical development of models describing the plastic deformation of UFG and bimodal structures. The bimodal structure was observed to delay the nucleation of stress concentrations at the boundaries between polygonal ferrite and bainite or martensite. This delay can also substantially reduce the propagation of cracks induced by dynamic loading, with an order of magnitude decrease higher than that observed in homogeneous structures. The presented findings may provide insights for designing impact-tolerant gradient structures with excellent dynamic properties.