Dynamic Deformation Properties of Medium-Mn Multi-phase Steels Containing Retained Austenite

We investigated the dynamic tensile properties of 4, 5, 6-mass%-Mn-containing low carbon steels with multi-phase microstructures containing retained austenite. The five materials used were classified into two groups. The first group of materials, with around 10% of retained austenite, showed normal strain rate dependence of yield strength (YS) and tensile strength (TS) as in conventional high strength steels. The second group of materials, containing 25-36 % of retained austenite and exhibiting LUders elongation, showed also normal strain rate dependence of YS and flow stress at Luders deformation, but TS varied in a complex manner. Among the second group, in the 4 Mn steel, TS was nearly constant at strain rates below 1 sH and increased slightly at higher strain rates. In the 5 and 6 Mn steels, TS once decreased up to the strain rate of 1 or 10 and then began to increase at higher strain rates. These behaviors were discussed in terms of temperature rise during plastic deformation and thermal stability of retained austenite. In the 4 Mn steel with relatively unstable retained austenite, almost all the austenite transforms regardless of strain rate. In the 5 and 6 Mn steels, where the retained austenite is moderately stable, its martensitic transformation is suppressed due to temperature rise, resulting in the decrease in TS at relatively low strain rates. At higher strain rates, the increase in flow stress prevails and TS begins to increase.