5th International High Manganese Steel Conference - HMNS 2022

Medium-Mn steels belonging to the third generation of Advanced High Strength Steels (AHSS) are the most perspective lightweight materials for body-in-white automotive parts. These steels containing between 3 and 12 wt.% of Mn show a beneficial combination of high strength at sufficient ductility. Their excellent mechanical properties are attributed to the substantial fraction of metastable austenite, which is able to transform into martensite during cold straining, resulting in a high work hardening rate and necking retardation. Therefore, the mechanical properties of medium Mn steels are directly related to the fraction and mechanical stability of retained austenite.
Typically, medium-Mn steels are manufactured through a single-step intercritical annealing process, resulting in a duplex microstructure consisting of austenite and ferrite. Despite many beneficial properties, such multiphase microstructure has some disadvantages related to the high difference in the hardness of neighboring phases resulting in their poor stretch-flangeability and limited hole expansion ratio (HER). Limiting these problems is possible due to the use of novel double-step intercritical annealing allowing the replacement of a significant part of soft ferrite by low-C martensite.
Temperature and time of intercritical annealing are the key parameters that affect the fraction and mechanical stability of retained austenite. However, heating and cooling rates have also an impact on the microstructure evolution. Therefore, the present study concerns the influence of heating and cooling rates during the second intercritical annealing step carried out at 800°C and 850°C on the microstructure of 0.16C-5Mn-1.6Al-0.2Si steel. Microstructure analysis was performed by means of scanning electron microscopy and electron backscatter diffraction techniques.