Comparative Study on Austenite Decomposition and Cu Precipitation During Continuous Cooling Transformation

Abstract

The nature of Cu precipitation during continuous cooling of austenite is an essential issue in thermomechanical processing of Cu-bearing high-strength low-alloyed steel. The Cu precipitation behaviors and microstructural evolution are comparatively studied at two representative cooling rates of 0.1 and 10 K/s using dilatometry, optical microscopy, transmission electron microscope, and atom probe tomography. The microstructure developed at the cooling rate of 0.1 K/s contains a variety of microconstituents of polygonal ferrite, acicular ferrite, bainite, martensite, and retained austenite, which is attributed to the varying concentrations of untransformed austenite and consequently changing austenite decomposition kinetics. The Cu precipitation occurs only in association with polygonal ferrite transformation by interphase precipitation mechanism. No Cu precipitates are detected in acicular ferrite, retained austenite, and martensite. At the cooling rate of 10 K/s, the microstructure is dominated by acicular ferrite dispersed with the C-Ni precipitates that are assumed to be formed on the retained austenite. The Cu precipitation generally occurs from aging because of the high temperature and supersaturated austenite in association with acicular ferrite transformation. The nature of Cu precipitation in different cooling rates greatly depends on the initial composition or the extent of solute enrichment in the prior untransformed austenite and the associated phase transformation mechanisms, resulting in the differences in sizes, morphology, and compositions of Cu precipitation.