5th International High Manganese Steel Conference - HMNS 2022

During the last years, Quenching and Partitioning (Q&P) steels have gained strong interest in the steel industry due to their promising lightweight potential and crashworthiness. With their microstructure consisting of tempered martensite (α’’) and C-enriched retained austenite (RA), they are characterized by balanced ductility characteristics, which close the gap between the conventional Dual Phase (DP) and Complex Phase (CP) steels.
The aim of this work was to investigate the influence of the microstructure on the ductility of several lean medium Mn Q&P steels. Therefore, three chemical compositions with varying C-contents between 0.10 and 0.20 wt-%, 4.0 wt-% Mn and 1.5 wt-%Si were thoroughly examined with regard to their structure-properties relationship. To assess the formability behavior of the investigated steels, parameters derived from tensile testing were utilized. In particular, the true uniform strain (εu,true) was used to describe the global ductility, whereas the true thickness strain (ε3,true) was determined as a characteristic value for the local one.
The results clearly demonstrated a strong effect of the RA fraction and its mechanical stability on the ductility behavior of lean medium Mn Q&P steels. Generally, the optimum exploitation of the Transformation Induced Plasticity (TRIP) ensured high strain-hardening rates and therefore favored high global ductility. However, the strain-induced martensitic transformation resulted in increasing heterogeneity in the microstructure, remarkably impairing the local ductility. By an increase in C larger RA fractions could be stabilized, which shifted the ductility characteristics from rather local towards global. For this reason, a medium C-content of 0.15 wt-% was found to result in the optimum microstructure leading to the desired balanced formability in combination with an excellent strength-ductility combination.