Design and high strain rate response of a nanostructured bainitic steel for enhanced austenite stability

This study investigates the microstructural evolution and mechanical response of a high-silicon nanostructured bainitic steel designed with optimized Mn and Ni contents to enhance the stability of retained austenite (RA). A combination of thermodynamic modeling (MUCG83) and advanced processing techniques, including electro-slag remelting and controlled austempering, was employed to tailor the transformation behavior. Particular attention is given to the synergistic effects of Mn and Ni, which promote carbon enrichment and increase stacking fault energy, thereby improving the mechanical and thermal stability of RA. Quantitative X-ray diffraction (XRD) analysis reveals that prolonged austempering results in a reduction in carbon content of RA from 1.73 wt.% to 1.51 wt.% while maintaining nearly constant volume fractions, suggesting carbide precipitation and carbon partitioning. Tensile tests and Split-Hopkinson Pressure Bar (SHPB) experiments show a remarkable balance of strength and ductility, with evidence of transformation-induced plasticity (TRIP) through stress- and strain-assisted martensitic transformation mechanisms. The high strain rate behavior further demonstrates a 37.5% drop in RA and a fivefold increase in energy absorption, attributed to accelerated transformation kinetics under adiabatic conditions. The findings provide insight into the interaction between alloying elements, heat treatment, and dynamic phase stability, and offer guidance for the design of advanced steels for structural applications involving high strain rates.