5th International High Manganese Steel Conference - HMNS 2022

While excellent compromise of strength and ductility of Medium Mn third generation steels are well known and deeply studied, the understanding of intergranular brittleness observed at different stages of the manufacturing process remains a major challenge for the industrialization of these new steels. Recent studies carried out at ArcelorMittal on a third generation Medium Mn 0.15C-4Mn-1.5Si steel have highlighted different conditions favoring such a brittleness:
1) After tempering: As already well reported in the literature, temper embrittlement of martensite is associated with the segregation of certain impurities to grain boundaries in the 300°C-650°C range. It is especially shown here that the phenomenon is reversible, and that formation of stable retained austenite during tempering might be very beneficial through its effect on crack blunting and on segregation.
2) After quenching from full austenitic region: Despite a lower segregation in prior austenite grain boundaries compared to that of ferrite, the higher martensite strength could lead to a large embrittling effect, especially for lower soaking temperature.
3) After overaging treatment: Among various overageing conditions tested, this embrittlement has only been observed when bainite is partially formed during overageing. Closer look at fracture surfaces highlighted a full interface decohesion at the bainite-fresh martensite interfaces. It is thought that it is a consequence of low interface strength and high strength mismatch between bainite and fresh martensite. It is supposed that the low interface strength is a consequence of segregation occurring during the phase transformation (so called solute-drag). Ductility is fully recovered after a tempering treatment.