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

Abstract

An ultra low carbon steel was subjected to two stages of forging operation and subsequently cooled at different cooling rates from the same finish forging temperature. Variation in microstructures and corresponding mechanical properties at different cooling rates were studied Microstructure obtained at the cooling rates of 35°C/s and 1.15°C/s was mainly bainitic ferrite along with granular shaped highly dislocated ferrite. Higher strength values were obtained due to highly dislocated bainitic ferrite along with fine microalloying carbide and carbonitride precipitates. The cooling rate of 0.68°C/s engenders polygonal / quasi polygonal ferrite along with a few coarse ferrite laths and the strength value drops due to formation of less dislocated polygonal ferrite. High impact toughness values at ambient and sub-ambient temperatures were obtained.