Hole expansion of retained austenite containing CMnSi bainitic/martensitic steels

Abstract

Austempering and Quenching & Partitioning steels are two processing approaches that may produce third generation advanced high strength steel properties, which are attractive to the automotive industry. Three 0.25C-2Mn-(1.5, 0.6, and 0.1)Si steels were austenitized and subsequently isothermally held at temperatures of 200-450 °C for 10-300 s. Significant fractions of retained austenite with differing morphologies, namely blocky retained austenite following austempering and fine film-like austenite following one-step Q&P were observed with X ray diffraction, Magnetic Saturation, Secondary Electron Imaging, and Electron Backscatter Diffraction. Tensile and hole expansion tests were conducted and correlated to strain hardening and microstructure rotation angle in the shear affected zone adjacent the sheared edge. At a given strength level, one-step Q&P produced superior hole expansion properties when compared to bainitic and mixed bainite/martensite microstructures.