Continuous cooling transformations and microstructures in a low-carbon,high-strength low-alloy plate steel

A continuous-cooling-transformation (CCT) diagram was determined for a high-strength low-alloy plate steel containing (in weight percent) 0.06 C, 1.45 Mn, 1.25 Cu, 0.97 Ni, 0.72 Cr, and 0.42 Mo. Dilatometric measurements were supplemented by microhardness testing, light microscopy, and transmission electron microscopy. The CCT diagram showed significant suppression of polygonal ferrite formation and a prominent transformation region, normally attributed to bainite formation, at temperatures intermediate to those of polygonal ferrite and martensite formation. In the intermediate region, ferrite formation in groups of similarly oriented crystals about 1 μm in size and containing a high density of dislocations dominated the transformation of austenite during continuous cooling. The ferrite grains assumed two morphologies, elongated or acicular and equiaxed or granular, leading to the terms “acicular ferrite” and “granular ferrite,” respectively, to describe these structures. Austenite regions, some transformed to martensite, were enriched in carbon and retained at interfaces between ferrite grains. Coarse interfacial ledges and the nonacicular morphology of the granular ferrite grains provided evidence for a phase transformation mechanism involving reconstructive diffusion of substitutional atoms. At slow cooling rates, polygonal ferrite and Widmanstätten ferrite formed. These latter structures contained low dislocation densities and e-copper precipitates formed by an interphase transformation mechanism.