Austenitic twinning-induced plasticity (TWIP) steels, which rely on high manganese (Mn)
contents, provide great potential in applications for structural components in the automotive
industry. These steels exhibit an excellent combination of tensile strength and ductility, e.g.
steels with tensile strengths above 1000MPa can present ductilities as high as 50%. These
outstanding properties can be reached because the high manganese contents, characteristic
of this steel grade, promote twinning as the main deformation mode.
The aim of this present work was to investigate the high temperature behavior, the static
recrystallisation kinetics and the evolution of microstructure of high Mn steels when submitted
to compression testing. In particular, the influence of homogenization heat treatment in the
temperature range (950°C–1250°C) was studied and some observations were made
regarding the evolution of plastic deformation at room temperature. Microstructures were
examined using optical and scanning electron microscopes. Three TWIP steels with different
compositions, i.e. 25%Mn, 20%Mn and 18%Mn, were examined and the results were used
to evaluate the effect of the composition, as well as the processing route, on their behavior at
high temperatures. In addition, double-hit tests and stress relaxation tests were performed on
one grade of these TWIP steels.
Metallographic evaluations performed after tensile tests at room temperature revealed that
the deformation mode of TWIP steels was twinning and that deformation twins were present
since the first stages of deformation. Heat treatment parameters were carefully chosen to
generate a new stress-free microstructure and, at the same time, limit the grain growth.
18%Mn and 25%Mn TWIP steels optimal temperature of heat treatment was 1100°C and it
was of 1200°C for the 20%Mn TWIP steel. All the heat treatments were one hour long. A
retarding effect of manganese on grain growth was detected.
Double-hit and stress relaxation tests performed on the 20%Mn TWIP steel were used to
describe the softening behavior at high temperature of TWIP steels. The recrystallization
kinetics was studied with stress relaxation tests and the influence of experimental
parameters was evaluated. The result showed that static recrystallization in TWIP steels is
little detectable by stress relaxation. However, metallographic observations revealed 100%
recrystallization when the sample was tested at 950°C at a strain of 0.2. Both strain and
temperature have little effect on recrystallization kinetics but seem to accelerate
recrystallization when they are increased. Stress relaxation tests are very interesting from an
experimental point of view because they imply a save of time and material; using the stress
relaxation technique, a single test is necessary to draw a complete graph of recrystallized
fraction as a function of time, whereas several tests would be necessary with double-hit
testing.
