Heterogeneous Multiphase Microstructure Formation through Partial Recrystallization of a Warm-Deformed Medium Mn Steel during High-Temperature Partitioning

• Published in: Materials Vol. 15; no. 20; p. 7322
• Main Authors: Sadeghpour, Saeed, Javaheri, Vahid, Somani, Mahesh, Kömi, Jukka, Karjalainen, Pentti
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 19.10.2022; MDPI
• Subjects: Annealing; Carbon; Cold; Crystal defects; Deformation; Ductility; Ferrite; Grain size; Heat resistant steels; High temperature; Hot rolling; Manganese steels; Mechanical properties; medium Mn steel; Microstructure; Morphology; Multiphase; partial recrystallization; Partitioning; Precipitation hardening; Precipitation hardening steels; Quenching; Recrystallization; Retained austenite; Stacking fault energy; Steel; Strain hardening; Temperature; Tempered martensite; Thermal simulators; warm deformation; Yield stress
• ISSN: 1996-1944; 1996-1944
• DOI: 10.3390/ma15207322

A novel processing route is proposed to create a heterogeneous, multiphase structure in a medium Mn steel by incorporating partial quenching above the ambient, warm deformation, and partial recrystallization at high partitioning temperatures. The processing schedule was implemented in a Gleeble thermomechanical simulator and microstructures were examined by electron microscopy and X-ray diffraction. The hardness of the structures was measured as the preliminary mechanical property. Quenching of the reaustenitized sample to 120 °C provided a microstructure consisting of 73% martensite and balance (27%) untransformed austenite. Subsequent warm deformation at 500 °C enabled partially recrystallized ferrite and retained austenite during subsequent partitioning at 650 °C. The final microstructure consisted of a heterogeneous mixture of several phases and morphologies including lath-tempered martensite, partially recrystallized ferrite, lath and equiaxed austenite, and carbides. The volume fraction of retained austenite was 29% with a grain size of 200–300 nm and an estimated average stacking fault energy of 45 mJ/m2. The study indicates that desired novel microstructures can be imparted in these steels through suitable process design, whereby various hardening mechanisms, such as transformation-induced plasticity, bimodal grain size, phase boundary, strain partitioning, and precipitation hardening can be activated, resulting presumably in enhanced mechanical properties.