Influence of Intercritical Annealing on Microstructure and Mechanical Properties of a Medium Manganese Steel

摘要

Medium manganese steels which belong to the 3rd generation advanced high strength steels are a promising material for the automotive industry due to their remarkable strength-ductility-combination enabling to reduce material and production costs. Today it is well established that the mechanical properties are strongly dependent on the intercritical annealing parameters which lead to the formation of an ultra-fine grained multi-phase microstructure. After cold-rolling the microstructure contains deformed α' martensite which transforms into recrystallized globular-shaped ferrite and austenite during intercritical annealing. This microstructure causes a pronounced yield point elongation in tensile tests due to the soft ferrite grains. The present work demonstrates that a Fe-12Mn-3Al-0.05C medium manganese steel can be produced by an adapted hot forming route showing a martensitic matrix with embedded reversed austenite islands after intercritical annealing in the cold-rolled material. After intercritical annealing at 555 °C for 1 h the cold strip exhibits a volume fraction of 15.7 % reversed austenite, an ultimate tensile strength (UTS) of 815 MPa and a total elongation of 23 % without observing a yield point elongation.