Effects of annealing temperature on the microstructure and mechanical properties of ultrafine grained medium-manganese TRIP steel

To study the feasibility of producing automobile steel from medium-manganese TRIP steel by continuous annealing processes, the effects of annealing temperature on the microstructure and mechanical properties of low-carbon medium-manganese steel were investigated on a heat treatment system CCT-AY-II for thin steel sheet at 590 to 710℃. The microstructure and components of the steel during annealing were characterized by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy. The amount of retained austenite in the steel was determined by X-ray diffraction analysis. The mechanical properties of the steel were investigated by uniaxial tensile testing. Experimental results show that after holding for 3 min, the amount of retained austenite first increases and then decreases with increasing annealing temperature. The elongation and the product of strength and elongation reach their maximum values of 21.3% and 28 GPa·% for the steel annealed at 650℃, respectively, and the tensile strength is 1330 MPa. Ultra-fine grain structures are obtained by recovering for the martensitic matrix and by twinning for retained austenite. It is believed that the high plasticity is provided by the TRIP effect of metastable austenite and ultra-fine grained ferrite or martensite together. A zig-zag shape appears on the true stress-strain curve. The work hardening exponent of the steel is much larger than that of traditional TRIP steel at the relatively stable stage.