Effect of annealing time on microstructure and mechanical properties of a cold rolled medium manganese steel

Effects of annealing time on microstructure and mechanical properties of cold rolled Fe-0.14C-5Mn steel annealed at 650 ℃ were investigated.Microstructure evolution of the steel during annealing process was characterized by means of scanning electron microscopy.The amount of retained austenite in the steel was determined by X-ray diffraction analysis.Mechanical properties of the annealed steel were studied by tensile tests.It is found that the austenite reverted transformation takes place,which results in more than 20% austenite phase in the ultrafine grained matrix after annealing at 650 ℃ for 1 min.Tensile strength(Rm) of the steel increases and its yield strength(Rp0.2),decreases with increasing the annealing time.Both elongation to failure(A) and the product of tensile strength to elongation to failure(Rm×A) first increase and then decrease.The maximum values of A(46%) and Rm×A(46 GPa%) are obtained for the steel annealed at 650 ℃ for 10 min.The high mechanical properties with strength Rm of 1000 MPa,elongation A of 46% and product of Rm×A of 46 GPa% are attributed to the enhanced TRIP effects by the large fraction of metastable austenite and the ultrafine grain size.