Medium Manganese Steels (MMnS) belong to the third-generation advanced high strength steels (3.G. AHSS) that have been developed because of their ecological and economic potential with a focus on automotive applications. Reducing weight, decreasing CO2 emission, simplifying processing are important drivers for their development.
This presentation deals with the interaction of chemical composition, processing, microstructure development and mechanical properties with a special focus on forging applications. The chemical composition range of Mn is from 4 to 10 weight-%, of C from 0.1 to 0,4 %, and also considers additions of Al, Cr, Si, V, Nb, Mo and B. The process steps discussed in detail include continuous casting, annealing, controlled cooling and tempering treatments. The special behavior of these steels during production as well as the characteristic microstructure and their particular mechanical properties are worked out.
In detail, the processability of MMnS is discussed in light of their hot ductility behavior. The findings indicate the prime role of precipitation, phase transformation and extension or shift of solidification intervals induced by the alloying concepts in controlling the hot ductility. It turns out that the formation of complex AlN and MnS precipitates as well as δ-ferrite solidification deteriorates the high temperature ductility.
Furthermore, the concept of MMnS is used to develop air-hardening forging steels. The alloy design and the heat treatment parameters have been varied with a focus on the prevention of Mn embrittlement as well as the formation of fine austenite grains during intercritical annealing. It is shown that the addition of B and Mo increase the impact toughness, although the effectiveness of each element varies depending on the heat treatment conditions. The impact toughness can be significantly increased by the introduction of a globular metastable austenitic phase. Compared to the reference quench+tempered steels an ultimate tensile strength level of more than 1300 MPa and an improved cyclic strength can be achieved. Thus, via a combined material, process and geometry optimization, these ductile air-hardening steels offer the possibility to save energy and CO2 emissions both by shortening the heat treatment and by light weighting the components. The critical aspects in controlling microstructures in MMnS are explained with respect to industrial applications.