Effect of cooling rate on intragranular ferrite formation in La deoxidized steel

The microstructures of La deoxidized steels obtained at different cooling rates were investigaled by optical microscopy and scanning electron microscopy coupled to an energy dispersive X-ray spectrometer. The continuous cooling transformation diagram of the tested steel was also drawn in this paper. It is found that in the cooling rate range of 4 to 10℃·s^-1, intragranular ferrite can form in the steel. While at the cooling rate of 8℃·s^-1, the microstructure mainly consists of fine and dispersed intragranular acicular ferrite (IAF). The lattice disregistry between lanthanum-containing inclusions and α-Fe is small; especially, this value of La₂O₂S is 0.2%. These kinds of inclusions can induce IAF nucleation and sympathetic nucleation, and also make grains refined.