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 Ms.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.