High strength Fe-Mn-(Al, Si) TRIP/TWIP steels development - properties - application

Deformation twinning, martensitic phase transformation and mechanical prope rties of austenitic Fe-(15-30) wt.%Mn steels with additions of aluminium an d silicon have been investigated. It is known that additions of aluminium i ncrease the stacking fault energy gamma(fcc) and therefore strongly suppres s the gamma --> epsilon transformation while silicon decrease gamma(fcc) an d sustains the gamma --> epsilon transformation. The gamma --> epsilon phas e transformation takes place in steels with gamma(fcc) less than or equal t o 20 mJ/m(2). For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at different str ain rates and temperatures. The formation of twins, alpha- and epsilon- mar tensite during plastic deformation was analysed by optical microscopy, X-ra y diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP ("transfo rmation induced plasticity") and TWIP ("twinning induced plasticity") steel s exhibit high flow stress (600-1100 MPa) and extremely large elongation (6 0-95%) even at extremely high strain rates of about 10(3) s(-1). Recent tre nds in the automotive industry towards improved safely standards and a redu ced weight as well as a more rational and cost effective manufacturing have led to great interest in these high strength and "super tough" steels. (C) 2000 Elsevier Science Ltd. All rights reserved.