Evolution of microstructure in an ultra‐low carbon Cu bearing HSLA forging

Over the last decade low carbon age hardenable steels have become a subject of interest for naval applications. In the present study an ultra‐low carbon Ni‐bearing steel alloyed with Cu was cast and processed by controlled forging and subsequently cooled in air, sand and water. A variety of multiphase structures have been observed because of different cooling rates. The slowest cooling rate has produced bainitic ferrite by ledge mechanism whereas lath‐ or plate‐type bainitic ferrite was predominant at the faster cooling rates. The volume fraction of retained austenite and/or martensite austenite constituents increased with decreasing cooling rate. Precipitation of finer micro‐alloying carbides and carbonitrides was suppressed by faster cooling. Slow cooling rates also resulted in the formation of ɛ‐Cu precipitates of varying sizes and shapes.