Mechanisms of ultrafine-grained austenite formation under different isochronal conditions in a cold-rolled metastable stainless steel

• Published in: Materials characterization 2016-08, Vol.118, p.129-141
• Main Authors: Celada-Casero, C., Huang, B.M., Aranda, M.M., Yang, J.-R., Martin, D. San
• Format: Article
• Language: English
• Subjects: AUSTENITE; AUSTENITIC STEELS; BACKSCATTERING; Cold rolling; Continuous heating; DISLOCATIONS; Electron back scatter diffraction; ELECTRON DIFFRACTION; ELECTRON MICROPROBE ANALYSIS; ELECTRON PROBES; GRAIN GROWTH; GRAIN REFINEMENT; HEATING RATE; Intermetallic compounds; MAGNETIZATION; MARTENSITE; MATERIALS SCIENCE; Metastable austenitic stainless steel; MICROSTRUCTURE; PHASE TRANSFORMATIONS; PRECIPITATION; Reversion mechanisms; SCANNING ELECTRON MICROSCOPY; STAINLESS STEELS; TEMPERATURE RANGE 0400-1000 K; Transformations; TRANSMISSION ELECTRON MICROSCOPY; Ultrafine-grained structures; VICKERS HARDNESS
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2016.05.014

The primary objective of this work is to obtain fundamental insights on phase transformations, with focus on the reaustenitization process (α′→γ transformation), of a cold-rolled (CR) semi-austenitic metastable stainless steel upon different isochronal conditions (0.1, 1, 10 and 100°C/s). For this purpose, an exhaustive microstructural characterization has been performed by using complementary experimental such as scanning and transmission electron microscopy (SEM and TEM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA), micro-hardness Vickers and magnetization measurements. It has been detected that all microstructural changes shift to higher temperatures as the heating rate increases. The reaustenitization occurs in two-steps for all heating rates, which is attributed to the chemical banding present in the CR state. The α′→γ transformation is controlled by the migration of substitutional alloying elements across the austenite/martensite (γ/α′) interface, which finally leads to ultrafine-grained reaustenitized microstructures (440–280nm). The morphology of the martensite phase in the CR state has been found to be the responsible for such a grain refinement, along with the presence of χ-phase and nanometric Ni3(Ti,Al) precipitates that pin the austenite grain growth, especially upon slowly heating at 0.1°C/s. [Display omitted] •Ultrafine-grained austenite structures are obtained isochronally at 0.1–100°C/s•The α′→γ transformation occurs in two steps due to the initial chemical banding•A diffusional mechanism governs the α′→γ transformation for all heating rates•The dislocation-cell-type of martensite promotes a diffusional mechanism•Precipitates located at α′/γ interfaces hinder the austenite growth