Precipitation and compositional changes in the structural phases of microalloyed automotive steels

A wide range of low and medium carbon steels are now employed for the manufacturing of various kinds of automobile parts. The steels exhibit considerable production and cost benefits as compared with conventional steels. In this work, the interphase precipitation of V(C,N) particles has been observed in the pro-eutectoid ferrite and ferritic lamellae of pearlite of the microstructure that is crucial for strengthening of medium carbon microalloyed steels. The compositional changes in the microstructure and various kinds of precipitation that are normally observed in microalloyed steels have been examined using transmission electron microscopic and parallel energy loss spectroscopic (TEM-PEELS) analysis techniques. Random and non-random dispersion of V(C,N) were observed in both the grain boundary and lamellar ferrite regions. PEELS analysis identified relatively higher concentration of nitrogen in fine non-randomly dispersed particles formed during austenite decomposition by an interphase precipitation mechanism, as compared with coarser particles randomly distributed on dislocations within the pro-eutectoid ferrite. The results are consistent with the sequence and respective growth mechanisms of the precipitates.