  Carbon is a well known austenite stabiliser and can be used to alter the
stacking fault energy and stability against martensitic transformation in
medium Mn steels, producing a range of deformation mechanisms such as the
Transformation Induced Plasticity (TRIP) or combined Twinning and
Transformation Induced Plasticity (TWIP $+$ TRIP) effects. However, the effect
of C beyond quasi-static tensile behaviour is less well known. Therefore, two
medium Mn steels with 0.2 wt\% and 0.5 wt\% C were designed to produce similar
austenite fractions and stability and therefore tensile behaviour. These were
processed to form lamellar and mixed equiaxed $+$ lamellar microstructures. The
low C steel had a corrected Charpy impact energy (KV\textsubscript{10}) of 320
J cm\textsuperscript{-2} compared to 66 J cm\textsuperscript{-2} in the high C
steel despite both having a ductility of over 35\%. Interface segregation, e.g.
of tramp elements, was investigated as a potential cause and none was found.
Only a small amount of Mn rejection from partitioning was observed at the
interface. The fracture surfaces were investigated and the TRIP effect was
found to occur more readily in the Low C Charpy specimen. Therefore it is
concluded that the use of C to promote TWIP$+$TRIP behaviour should be avoided
in alloy design but the Charpy impact performance can be understood purely in
terms of C in solution.
