Particularities of the formations of bainite and martensite/austenite phase in low carbon low alloy steels during continuous cooling

Abstract The industrial X80 steel chemistry has been modified by increased amounts of Mn, Cr and a new leaner base chemistry (0Ni-0Cu-0.04Nb) has been used with carbon contents of ± 0.04 and 0.07 wt.%. Continuous-cooling-transformation diagrams were obtained by deformation dilatometry to determine the effects of steel compositions on phase transformations and microstructure evolution with cooling rates in the range of 0.5–130 Ks−1. The effects of alloying on bainite transformation, its kinetics and the formation of martensite/austenite constituents were determined. Thermodynamic analysis is carried out for the modified chemistries and it shows that growing bainite laths stimulate the decomposition of residual austenite by strain-induced martensite transformation at a later stage of the overall bainite transformation.