Influence of isothermal bainitic processing on the mechanical properties and microstructure characterization of TRIP steel

The mechanical properties of transformation induced plasticity (TRIP) steel are strongly affected by the conditions of isothermal bainitic processing. The multiphase microstructure of TRIP steel under different conditions of isothermal bainitic processing was investigated using OM, SEM, XRD and TEM. The volume fraction of retained austenite and the carbon content in austenite were determined quantitatively using X-ray diffraction patterns. The relationship between mechanical properties and isothermal bainitic processing parameters was investigated. The stability of retained austenite was analyzed by the volume fraction of retained austenite and the carbon content in retained austenite. The experimental results show that the multiphase microstructure consists of ferrite, bainite and metastable retained austenite. To obtain good mechanical properties, the optimal conditions of isothermal bainitic temperature and holding time are 410-430°C and 180-240 s, respectively. After isothermal bainitic processing under the optimal conditions, the corresponding volume fraction of retained austenite is 5vol%-15vol%, which can provide enough retained austenite and plastic stability for austenite with high carbon content.