Structure and properties of an ultra low carbon HSLA forging

An ultra low carbon HSLA steel was subjected to a two, stage-forging operation followed by different post-cooling techniques. The higher strength value, obtained at a faster cooling rate of 35°C/s, is due to a highly dislocated acicular ferrite structure along with fine precipitation of microalloying carbides and/or carbonitrides. At a slower cooling rate of 1.15°C/s, the strength value drops with an increase in ductility due to the larger volume fraction of less dislocated polygonal ferrite structure. The strength value remained almost unchanged with a further decrease in cooling rate to 0.68°C/s due to the formation of polygonal ferrite microstructure predominantly. At the slowest post-cooling rate, a high impact toughness value was obtained at ambient and at -40°C testing temperature due to fine grained polygonal ferrite microstructure. At all post-cooling conditions, the change in impact toughness value at ambient and at -40°C temperature was found to be negligible due to the ultra low carbon content of the steel.