AbstractsEarth & Environmental Science

"Lightweight duplex steels with combinations of either bainite, acicular ferrite, and austenite or martensite and austenite were investigated as third generation advanced high strength steels targeted for automotive applications. Large additions of manganese (>13 wt%) and carbon (<0.2wt%) were employed to stabilize the austenite phase. Silicon additions between 1 and 2 wt% were added to suppress cementite fonnation. Strength and ductility were increased while density was decreased with aluminum additions between 2.4 and 5.5 wt% to the steel. This research addressed the dependence of alloying on microstructures and mechanical behavior for high manganese and aluminum duplex steels that were cast and subsequently hot rolled. Duplex steels with different volume fractions of primary [delta]-ferrite were used to study the crystallography of austenite formed during the peritectic reaction. Solute profiles across the peritectic interface showed aluminum segregated near the interface which promoted bainitic ferrite formation. Thermal treatments were used to manipulate the concentration and type of oxides and the ferrite plate density was found to correlate with inclusions of low misfit in steels with austenite grain size of 16.5 f!m. A steel with bainite and acicular ferrite produced an ultimate tensile strength of 970 MPa and elongation of 40%. The mechanical properties depended on the strengths and size of the microstructural constituents. Work hardening behavior was examined in a steel exhibiting multiple martensitic transformation induced plasticity ([gamma]-austenite->[epsilon]martensite->[alpha]-martensite). A strain hardening exponent as high as 1.4 was observed with ultimate tensile strength and elongation as high as 1,165 MPa and 34%" – Abstract, leaf iv.