Superdense microbands strengthening of textured low alloy ferritic steel

Microstructure evolution of Ti-Mo microalloyed ferritic steel was studied via warm rolling process. Results revealed that rolling at 500 °C (R3) mainly facilitated the formation of superdense microbands (∼200 nm in spacing), especially in the γ textured region. Average spacing of microbands (MBs) increased with temperature and it turned to be about 850 nm when rolling at 650 °C (R2). As rolling temperature further increased to 700 °C (R1), average ferritic grain size was refined to near 1.9 μm whereas no MBs appeared. Additionally, due to the preexisting superdense MBs, ultimate tensile stress of R3 increased to 1041 MPa and the elongation kept high at 22.9% simultaneously. MBs are characterized by high density dislocation walls and formed through the activation of {112}<111> slip systems, which is induced by the pile up of dislocations during grain boundary sliding. In that process steps appear in the grain boundary and gradually heighten with the extension of MBs. Large dynamic recovery rate could slack the dislocation pile up rate and increase the spacing of MBs. Moreover, preexisting of superdense MBs sheds new insights on the development of high strength ferritic steels with excellent combination of strength and ductility.