Basic direct heat treated (DHT) steel has a microstructure consisting of ferrite and pearlite, and the fatigue strength of such steels is not sufficiently high to meet the specifications for most highly stressed automotive parts. The results of early investigations of fatigue strength revealed the weakness of the ferrite microstructure: fatigue cracks initiate from ferrite and propagate until final fracture.
The influence of microalloying elements on fatigue strength of ferrite and pearlite microstructure has been investigated. The influence of vanadium and carbon contents on proof strength of microalloyed DHT steels has been measured. Studies show that vanadium is the most effective microalloying element, and it is widely used to improve the strength of microalloyed steels, as shown here in Fig. 9.
https://dl.asminternational.org/handbooks/book/9/chapter/113021/Forge-and-Direct-Heat-Treatment-Processes-and-Technologies
