Acicular ferrite microstructures and mechanical properties in a low carbon wrought steel

In this work, the formation of acicular ferrite in a low carbon wrought steel has been investigated. After austenitizating at 1250 degrees C for 45 minutes, the steel has been subjected to isothermal heat treatments in the range of 550-350 degrees C for different times. The microstructure obtained is mainly composed of acicular ferrite. Widmanstatten and allotriomorphic ferrite have also been found nucleating at the prior austenite grain boundaries. The volume fraction of these phases decreases remarkably when the isothermal treatment temperature is diminished. It has been observed that acicular ferrite is nucleated at non-conventional metallic inclusions such as multiphased inclusions composed of Al, Si and Mn oxides and MnS inclusions, which are covered, in both cases, by a shell of a hexagonal CuS phase. The mechanical properties of the steel in the as-rolled condition and after the generation of acicular microstructures have been measured showing the improvement obtained with acicular ferrite.