Intrinsic Increment of Plasticity Induced by TRIP and Its Dependence on the Annealing Temperature in a Lean Duplex Stainless Steel

Recently, advanced lean duplex stainless steels (LDXs) with exceptionally good tensile properties by transformation-induced plasticity (TRIP) have been developed to respond to the skyrocketing raw material cost. In these new alloys, TRIP in the metastable austenite phase is expected to dominate overall deformation of the steels. Solution annealing, as a critical step of production processing, affects the austenite characteristics in LDXs, such as volume fraction and mechanical stability of austenite, which in turn influences its TRIP behavior. In order to further develop advanced LDXs, an assessment in the plastic increment of TRIP and its dependence on solution treatment are necessary. In this work, the tensile deformation test of a LDX which was annealed in the range of 1000 similar to 1200 degrees C was carried out on a Gleeble-3800 machine. The microstructural mechanism of work hardening characteristics was characterized by TEM, and the saturation of strain-induced martensite (SIM) under different conditions was calculated by XRD. Some quantitative indicators which can characterize the plastic increment of TRIP were proposed, including apparent plastic increment (Delta e), average plastic increment ((Delta e) over bar) induced by unit volume SIM and intrinsic plastic increment (Delta e*) related only to mechanical stability of austenite. Meanwhile, their dependences on annealing temperature were discussed. The results show that SIM can develop in two ways of gamma ->epsilon ->alpha' and gamma ->alpha' whereby the work hardening of the LDX exhibit a "three-stage" characteristic. There is a critical deformation temperature (M-d) where the TRIP is absent at every annealing temperatures. The higher the annealing temperature is, the smaller the M-d, and the Delta e are. As annealing temperature increases, (Delta e) over bar increases, while Delta e* decreases, indicating a fact that the more stable the austenite is, the smaller the intrinsic plastic increment of TRIP is. In addition, both (Delta e) over bar and Delta e* show a linear relationship with the austenite stability coefficient (k).