Most first-generation direct heat treated (DHT) steels were alloyed with small amounts of vanadium, but several other alloying elements were added to improve toughness. The ferrite matrix is strengthened by alloying element effects and the precipitation of vanadium carbonitride.
Further additional improvements of strength and toughness of ferrite and pearlite microalloyed steels were achieved by the use of microalloying technologies developed for high-tensile-strength sheet steels.
The basic 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 first important factor in increasing the mechanical properties of microalloyed ferrite and pearlite DHT steel is the improvement of ferrite strength. Figure 7 shows the limit of microcrack initiation.
For more information, click on the link below (subscription required). Then scroll down the Chapter Contents column on the left of the page, and click on Microalloyed Steels for Direct Heat Treatment Technology. Then scroll to “Microstructure Control Technologies for Microalloyed DHT Steels.”
