Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, Al 1.3, and Nb+V 0.13 (mass%) with annealed martensite (TAM steel) were investigated using optical microscopy, field emission gun scanning electronic microscope (FEG SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and mechanical testing. The microstructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix (TPF steel), the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain hardening behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.