TWIP steels have a high manganese content (17-24%) that causes the steel to be fully austenitic at room temperatures. A large amount of deformation is driven by the formation of deformation twins. This deformation mode leads to the naming of this steel class. The twinning causes a high value of the instantaneous hardening rate (n-value) as the microstructure becomes finer and finer. The resultant twin boundaries act like grain boundaries and strengthen the steel. Figure 2-16 shows the as annealed microstructure for a TWIP steel.
Figure 2-16: Photomicrograph of TWIP steel as annealed
TWIP steels combine extremely high strength with extremely high stretchability. The n-value increases to a value of 0.4 at an approximate engineering strain of 30% and then remains constant until both uniform and total elongation reach 50%. The tensile strength is higher than 1000 MPa. Engineering and true stress-strain curves for TWIP steel grades are located in Figure 2-17 in the Advanced High-Strength Steels Application Guidelines.
Current production grades of TWIP steels and example automotive applications:
TWIP 500/900 A-Pillar, wheelhouse, front side member
TWIP 500/980 Wheel, lower control arm, front and rear bumper beams, B-pillar, wheel rim
TWIP 600/900 Floor cross-member, wheelhouse
TWIP 750/1000 Door impact beam
TWIP 950/1200 Door impact beam