Influence of isothermal transformation temperature on microstructure and mechanical properties of a TRIP steel containing Nb

The influence of isothermal transformation temperature on microstructure and mechanical properties of a cold rolled TRIP steel sheet with the composition of Fe-0.15C-1.46Si-1.56Mn-0.06Nb was investigated by mean of optical microscopy and X-ray diffraction.The results show that the optimum heat treatment process for the steel is austenizing in intercritical region at 840 ℃ for 180 s and then isothermal at 420 ℃ for 240 s for bainite transformation.The microstructure of steel sheet under this heat treatment is composed of 72% ferrite,20% bainite and 8% retained austenite,and the maximum product of strength and elongation,which is 2.5×10~4 MPa·%,is also obtained by this heat treatment process.The product of strength and elongation of the steel can be increased by increasing holding time properly at 840 ℃,due to the increase of volume fraction of retained austenite and carbon content in retained austenite.