Tensile property and microstructure of Fe-22Mn-0.5C TWIP steel

The tensile property and microstructure evolution of Fe-22Mn-0.5C TWIP steel under different annealing conditions was experimentally investigated. The matrix phase of this steel after annealing and tensile test is single austenite. The (111) peak is the most pronounced under as annealed condition, however, its intensity decreases significantly after stretched to fracture where (220) peak becomes the most dominant. The yield strength and tensile strength of Fe-22Mn-0.5C TWIP steel mainly decrease as annealing temperature increases, while the total elongation largely increases. The variation in the product of tensile strength and total elongation against temperature is related to the annealing time. The most obvious change in strength and elongation is located between 750 and 850 °C, due to the grain coarsening in the recrystallized matrix. The grain size and density of annealing twins in Fe-22Mn-0.5C TWIP steel increases with increasing anneal temperature. The recrystallized grains grow obviously as annealing temperature and time increase. The influence of annealing time changing by 15 min on grain growth is not as obvious as temperature changing by 100 °C under the current condition. The tensile strength of Fe-22Mn-0.5C TWIP steel decreases as grain size increases while the total elongation increases. The variation in strength and elongation against the square root of grain size follows Hall-Petch's law well. In order to get a good combination of strength and ductility, the grain size between 10 and 30 µm for TWIP steel is preferentially suggested, within which the product of strength and ductility can be easily adjusted to higher than 65 GPa%. High strength and ductility can be obtained in full-recrystallized Fe-22Mn-0.5C TWIP steel with fine-grained matrix. The high strength TWIP steel with tensile strength above 1030 MPa and total elongation about 60% can be prepared by annealing at 650-750 °C for 15-20 min. The high ductility TWIP steel with tensile strength around 900 MPa and total elongation beyond 70% can be manufactured by annealing at 850-950 °C for 5-10 min.