Unique microstructure formations during low-temperature partitioning after intercritical annealing in low alloy multi-phase TRIP steel and their mechanical behavior clarified by in-situ synchrotron X-Ray diffraction

Abstract

Strong attentions have been paid to low-carbon multi-phased steels showing transformation induced plasticity (TRIP) effect as good candidates for automotive applications due to their good mechanical balance between strength and ductility. However, details of complicated microstructural evolutions during thermo-mechanical processing and the roles of constituent phases on mechanical properties have not yet been fully clarified. In the present study, the formation process of multi-phased microstructures in a low alloy steel (Fe-0.2C-1.6Mn-1.4Si-1.0Ni-0.5Al: wt.%) during intercritical annealing in austenite + ferrite temperature (830 °C) and subsequent partitioning heat-treatment at lower temperature (400 °C) were systematically investigated. The phase fraction of ferrite (F), martensite (M), and retained austenite (A) significantly changed with increasing the holding time at 400 °C. It was observed that austenite transformed into ferrite during the heat-treatment at 400 °C to form newly formed ferrite having different characteristics. The ferrite in the final microstructures was categorized into four types (Type I, II, III and IV), based on their morphologies and other features. The ferrite newly formed at 400 °C (Type II, III, IV) were concluded to be formed by massive or bainitic transformation from their characteristics, i.e., morphologies, existence of surface relief, dislocations and misorientations, chemical compositions, and thermodynamic considerations. Long-range diffusion of substitutional elements did not occur in the phase transformation, but interstitial carbon diffused and enriched into austenite, resulting in large fractions of retained austenite at room temperature. Tensile mechanical properties of the obtained multi-phase steel were systematically clarified by in-situ synchrotron X-Ray diffraction. Tensile elongation increases with increasing the holding time at 400 °C, because the amount of retained austenite increased with the partitioning heat-treatment and they showed the TRIP effect through deformation-induced martensitic transformation during tensile deformation. On the other hand, the best strength-ductility balance was obtained in the specimen experienced the shortest heat-treatment at 400 °C.