Microstructure and Properties of 800 MPa Cold Rolled TRIP Steel

12 kind of annealing processes were worked out based on the CCT curves of the new developed 800 MPa (TRIP) steel which were determined by full automatic thermal simulation machine.The optimal heat treatment process was found out after the mechanical properties of testing steels annealed with different processes were obtained by tension tests.The microstructure of the steel treated with the optimal annealing process and the mechanical stability of the retained austenite were investigated.The results show that 800 MPa tensile strength could be obtained by all kinds of processes,the optimal comphrehensive mechanical properties (the biggest Rm×A value) could be obtained using following process: annealing at 830 ℃ for 120 s,then slow cooling to 700 ℃ at 20 ℃·s-1,and fast cooling to 400 ℃ at 40 ℃·s-1,after isothermal treating at 400 ℃ for 400 s,cooling to room temperature at 20 ℃·s-1.The microstructure of the steel with optimal annealing process was 50% ferrite,38% bainite and 12% retained austenite.The retained austenite grains mainly dispersed at ferrite grains boundaries,or the boundaries between ferrite and bainite grains,and a fraction of retained austenite located inside big ferrite grains.Most retained austenite phases of the testing steel transformed during 10%-20% tensile deformation.