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 epsilon-Cu precipitates of varying sizes and shapes.