A Study of Microstructure and Phase Transformations of Medium-Carbon Dual-Phase Steels

Abstract

Microstructure development in medium-carbon dual-phase steels depends on various phase transformations depending on initial microstructure and cooling. Dual-phase steels may contain small amounts of retained austenite and bainite as well austenite and ferrite. In this study, intermediate quenching applied to medium-carbon hot rolled steel. The microstructures were characterized using field emission scanning electron microscopy and focused ion beam scanning electron microscopy. Microstructural investigations show that two different types of martensite morphology can be found in microstructure as martensite island and martensite fibers. During the annealing, the nucleation of austenite may occur along the lath boundaries, in martensite laths, blocks and packets as well as prior austenite grain boundary. It is concluded that martensite islands were nucleated at prior austenite grain boundary, while martensite fibers nucleated at martensite laths. Granular bainite was also observed at the boundaries between martensite islands. DICTRA software was applied to understand formation of granular bainite by means of elemental distribution.