Supra-ductile and high-strength manganese-TRIP/TWIP steels for high energy absorption purposes

The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4 mass%) were investigated as a function of temperature (-196 to 400degreesC) and strain rate (10(-4) less than or equal to epsilon less than or equal to 10(-3) s(-1)). Multiple martensitic gamma(fcc) (austenie)-->epsilon(hcp)(MS) (hcp-martensite)-->alpha(bcc)(Ms) (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150degreesC at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Gamma(fcc) of the fcc austenite and the phase stability determined by the Gibbs free energy DeltaG(gamma-->epsilon). These factors are strongly influenced by the manganese content and additions of aluminum and silicon. The stacking fault energy Gamma(fcc) and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Gamma(fcc) and suppresses the gamma(fcc)-->epsilon(hcp)(Ms) transformation, whereas silicon sustains the gamma(fcc)-->epsilon(hcp)(Ms) transformation and decreases the stacking fault energy. At the critical value of Gamma(fcc)approximate to25 mJ/mol and for DeltaG(gamma)-->epsilon>0, the twinning mechanism is favored. At lower stacking fault energy of (Gamma(fcc)<16mJ/mol and for DeltaG(gamma-->epsilon)>0, martensitic phase transformation will be the governing deformation mechanism. The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.