Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test,scanning electron microscopy (SEM),transmission electron microscopy (TEM),X-ray diffraction (XRD) and thermodynamic calculation (TC).It was shown that the ferrite-based lightweight steels with 5 wt.％ or 8 wt.％ Al were basically composed of ferrite,austenite and κ-carbide.As the annealing temperature increased,the content of the austenite in the steel gradually increased,while the κ-carbide gradually decomposed and finally disappeared.The mechanical properties of the steel with 5 wt.％ Al and 2 wt.％ Cr,composed of ferrite and Cr7C3 carbide at different annealing temperatures,were significantly inferior to those of others.The steel containing 5 wt.％ Al,annealed at 820 ℃ for 50 s then rapidly cooled to 400 ℃ and held for 180 s,can obtain the best product of strength and elongation (PSE) of 31242 MPa · ％.The austenite stability of the steel is better,and its PSE is higher.In addition,the steel with higher PSE has a more stable instantaneous strain hardening exponent (n value),which is mainly caused by the effect of transformation induced plasticity (TRIP).When the κ-carbide or Cr7C3 carbide existed in the microstructure of the steel,there was an obvious yield plateau in the tensile curve,while its PSE decreased significantly.