Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Abstract

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 1017 m−3 when the dispersed austenite grain size is down to 1 µm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (Ms) temperature has been determined to be Ms (K)=273+545.8 · e−1.362wc(mass pct). A function describing the Ms temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the Ms temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the Ms temperature when the grain size is as small as 0.1 µm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the Ms temperature with respect to the TRIP steels.