Effect of Annealing Treatment on Retained Austenite Characteristics and Tensile Properties in TRIP-assisted Steel

A combination of elongation and high strength is achieved in process of a TRIP-assisted multiphase steel (Fe-Mn-Si-C) by intercritical annealing and then rapid cooling into bainite transformation temperature range to hold several minutes. This combination is caused by TRIP(TRansformation Induced Plasticity) of retained austenite. For this research, the TRIP-assisted steels with different chemical compositions were annealed for a range of variable time (5 min. ∼ 40 min.) in austenite single region to clarify the effect of annealing treatment condition on the mechanical property and retained austenite characteristics. As a result of microscopic observation (transmission electron microscopy), the morphology of retained austenite could be identified as two typer: a granular type and film type. Actually, suitable volume fraction of metastable austenite was a requisite for the good formability necessarily required as a principle mechanical property in automobile industry. For the case of HC steels showing a tensile strength of 860 MPa and a total elongation of 38 %, film typed reatained austenite could be observed between lath bainitic ferrite.