Effects of annealing temperature on the microstructure and mechanical properties of TRIP steel with annealed martensitic matrix

Special TRIP steel with annealed martensite matrix (TAM steel) was produced by fully quenching and annealing in the two-phase region with the C- Si- Mn chemical composition of common TRIP steel. The microstructure and mechanical properties of TAM steel annealed at different temperatures were investigated by thermal dilatometry, tensile testing, optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. It is found that the microstructure of TAM steel consists of a uni-form fine annealed martensite matrix and an interlath second phase of retained austenite and bainite/martensite, which contribute to lowering the matrix's hardness and the strength ratio of matrix to second phase as well as decreasing the dislocation density. As the an-nealing temperature rises, the mixed blocky microstructure of newly formed martensite/bainite gradually increases, while the lath-like morphology of annealed martensite progressively disappears. Excellent mechanical properties are obtained when the annealing tempera-ture is 780℃, with the tensile strength, the elongation, and the product of strength and ductility up to 1130 MPa, 20%, and 22600 MPa·%, respectively. When the annealing temperature is relatively low, retained austenite mainly exists between annealed martensite laths in film-form and is conducive to the occurrence of TRIP effect.