Comparison between the Lüders and Portevin–Le Chatelier bands in the low-strain-rate tensile testing of ultralow-carbon ferritic steel

In ferritic steels, solute carbon (C) causes two types of discontinuous stress fluctuations that are accompanied by local deformation bands in the stress–strain curves. One is the yield drop with the Lüders band at yielding, and the other is the serrated flow stress with Portevin–Le Chatelier (PLC) bands during the strain hardening stage, that is, the PLC effect. Lüders band and PLC bands can be explained by static strain aging and dynamic strain aging, SSA and DSA, respectively. These difference in strain aging mechanics distinguish the Lüders band and PLC bands and qualitatively explain when they appear in the stress–strain curve at the yielding and strain-hardening stages. Nevertheless, Lüders band and PLC effect occur in carbon steels at room temperature and 373–473 K, respectively. Therefore, fundamental difference between these bands remains unclear because it is difficult to compare them under the same tensile conditions. In this study, low-strain-rate tensile tests were performed on ultralow-carbon ferritic steel at ambient temperature to compare the bands under the same deformation conditions. In addition to the Lüders band, the formation and propagation of PLC bands were observed at strain rates lower than 1.0 × 10^-4 s^-1, and the PLC effect became more pronounced as the strain rate decreased and the carbon content increased. Furthermore, local strain analysis using digital image correlation revealed that the dislocation movement was much faster than C diffusion only in the Lüders band, which is attributed to the difference in the strain-aging mechanism.