  A Twinning Induced Plasticity (TWIP) steel with a nominal composition of
Fe-16.4Mn-0.9C-0.5Si-0.05Nb-0.05V was deformed to an engineering strain of 6\%.
The strain around the deformation twins were mapped using the 4D-STEM
technique. Strain mapping showed a large average elastic strain of
approximately 6\% in the directions parallel and perpendicular to the twinning
direction. However, the large average strain comprised of several hot spots of
even larger strains of up to 12\%. These hot spots could be attributed to a
high density of sessile Frank dislocations on the twin boundary and correspond
to shear stresses of 1--1.5 GPa. The strain and therefore stress fields are
significantly larger than other materials known to twin and are speculated to
be responsible for the early thickness saturation of TWIP steel nanotwins. The
ability to keep twins extremely thin helps improve grain fragmentation,
\textit{i.e.} the dynamic Hall-Petch effect, and underpins the large
elongations and strain hardening rates in TWIP steels.
