Kinetics and mechanisms of intragranular ferrite nucleation on non-metallic inclusions in low carbon steels

Isothermal heat-treatment was conducted to study the microstructures of intragranular ferrite nucleated on non-metallic inclusions in a low carbon steel. The dominant transformed products were grain boundary ferrite, intragranular polygonal ferrite, acicular ferrite and bainite in turn from high to low temperature (650–500 °C). The favorite temperature range for the occurrence of largest amount of acicular ferrite was about 580–620 °C in this study. The morphology of acicular ferrite and intragranular polygonal ferrite was also studied considering the axial ratio. Acicular ferrite and polygonal ferrite were measured with electron back scattering patterns to have different orientation relationship with austenite, Kurdjumov–Sachs (K–S) relationship of the former and possible random of the latter.