Continuous cooling transformation and structure of a clean microalloying steel

An ultra-low carbon micro-alloy ing clean steel containing Mn, Nb, Ti, B was prepared by induction furnace in laboratory. Total amount of impurities O, S, P and H in the steel is less than 0.0061%(mass fraction). Continuous cooling transformation behaviors of the clean steel and a commercial steel X60, whose composition is similar as the clean one, have been studied in the present work. The high purity in the clean steel results in the decrease of heterogeneous nucleation rate of a-phase during continuous cooling transformation, and the Bs temperature drops down obviously. But 70% deformation at 900 °C results in the rise of the Bs. Structure with grain size in micrometers exists in the clean steel specimens cooled at certain cooling rate. But no such fine structure was found in the commercial steel specimens. The results implied that the γ -> α transformation dynamics, especially the nucleation dynamics of α phase, in the clean steel is different from the commercial steel. It is likely that the new phase in very clean steel is mainly formed by homogeneous nucleation under certain conditions. The difference of γ -> α transformation nucleation mechanism between the clean steel and the commercial one resulted in their different structures.