Decomposition of Supercooled Austenite in Continuous Cooling Transformation Process of a Mn-Mo-Ni Low Alloy Steel

Abstract

Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni low alloy steel was evaluated by dilatometric measurements, light microscopy, electron backscatter diffraction, and microhardness testing and other methods. The results show that at the cooling rates of 1°C/s or below, ferrite initially formed and continuously rejecting C into the untransformed austenite, which transforms to C-rich lower bainite at lower temperature, resulting in ferrite-bainite dual-phase microstructures. At the cooling rates between 1 °C/s and 5 °C/s, the successive transformation products are bainite ferrite, upper bainite and lower bainite, and islands of carbon-enriched austenite transform to martensite (plus retained austenite) at low temperatures. The upper bainite and martensite dual-phase microstructures are formed at the range of 5 °C/s to 50 °C/s with a lower M_s. When cooling rates greater than approximately 50 °C/s, the microstructure are martensite and retained austenite at room temperature. With the increase of cooling speed, the residual austenite content increased before decreased at the cooling speed of 2 °C/s, which is probably associated with the incompleteness of bainite transformation.