Effect of cooling rate on structure and properties of an ultra-low carbon HSLA-100 grade steel

An ultra-low carbon HSLA-100 grade steel was subjected to two stage forging operation followed by different post-cooling techniques. Higher strength value obtained at a faster cooling rate of 35 °C/s is due to 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 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 predominantly polygonal ferrite microstructure. At slower post-cooling rates high impact toughness value obtained at ambient and at − 40 °C testing temperature is 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.