Formation of Ferritic Products during Continuous Cooling of a Cu-bearing HSLA Steel

摘要

A continuous cooling transformation (CCT) diagram was constructed by a combination of dilatometry and metallographic methods for a Cu-bearing HSLA steel, which is a low carbon low alloy variant of the ASTM A710 type structural steel. It was found that the decomposition of austenite was significantly depressed to lower transformation temperatures resulting in a prominent transformation region for bainitic structures, at temperatures intermediate between those of diffusional products and the displacive transformation to martensite. Polygonal and quasi-polygonal ferrite were observed to grow across and eliminate the prior austenite grain boundaries at relatively low cooling rates. At a cooling rate ranging from 0.35 to 20℃/s, the structure was characterised by a mixture of quasi-polygonal ferrite, Widmanstaetten side-plate ferrite, and bainitic structures associated with minor dispersed islands of martensite and/or retained austenite which were dark etching on preparation for optical microscopy. This microstructure develops by the following processes. The Widmanstaetten side-plate ferrite nucleates from the ferrite grain boundary allotriomorphs at the early stage of transformation, together with the bainitic ferrite plates which nucleate directly at the prior austenite grain boundaries. On further cooling, the neighbouring plates of Widmanstaetten and bainitic ferrite each tended to coalesce and the volume of untransformed austenite decreased and the shapes of the enclosed γ volumes evolved into residual islands between the ferrite plates. Provided the cooling rate was greater than 20℃/s, the bainitic ferrite plates nucleated directly at the prior austenite grain boundaries, and the plate morphology was revealed by regions of elongated retained austenite or its decomposition products. At the fastest cooling rate obtained by dilatometry (～375℃/s), the structure was largely characterised by a mixture of bainitic ferrite and martensitic packets surrounded by retained austenite films. Dilatometric and metallographic examination of the martensite and bainitic ferrite formed on rapid cooling failed to find a clear microstructural distinction between the two products. The packets of bainitic ferrite plates were generally nucleated directly from the prior austenite grain boundaries, whereas the martensite was characterised by thinner ferritic units with a higher dislocation density. There also appeared to be a larger number of variants of lath packets and apparent intragranular nucleation in the case of martensitic ferrite.