Experimental evidence for “double-edged sword” effect of the fibrous δ-ferrite on mechanical properties: a case study of Fe-5.95Mn-1.55Si-1.0Al-0.055C medium-Mn steel

An ingenious thermo-mechanical treatment process was utilized on a Fe-5.95Mn-1.55Si-1.0Al-0.055C medium-Mn steel to investigate the effect of fibrous δ-ferrites on mechanical properties. With a large enough cold-rolling reduction ratio (∼70%), the morphology of coarsened fibrous δ-ferrites is tailored into homogeneous distributed fragments, which provides a precondition to launch the present study. Direct experimental evidence has been obtained that fibrous δ-ferrites are a double-edged sword for mechanical properties: it improves ductility (by 3.5% ± 1.1%) but decreases tensile strength (by 65 ± 11.5 MPa). This can be related to the accommodation effect of dislocations within coarsened fibrous δ-ferrite grains as soft phase, which decreases the degree of dislocation pile-ups in martensitic matrix, resulting in the limited strengthening effect of dislocations. Additionally, despite significant directionality along rolling direction, the fibrous δ-ferrite grains do not result in anisotropic properties.