Microstructures and properties of direct-cooled microalloy forging steels

In comparison to conventionally processed quenched-and-tempered steels, direct-cooled microalloy steels offer the potential for significant cost savings. However, direct material substitutions have often been limited based on toughness considerations at the required hardness levels. In an effort to improve toughness, direct-cooled microalloyed forging steels have evolved from precipitation strengthened ferrite–pearlite steels to steels with non-traditional bainitic microstructures that may contain a significant amount of retained austenite. As a consequence of recent developments, the use of direct-cooled microalloyed steels has increased and there is current interest in the use of these steels processed to obtain higher hardness levels. In this paper, processing approaches for the production of direct-cooled forging steels are considered with an emphasis on features that control strength and toughness.