Mechanism of fatigue failure in interstitial-free and interstitial-free high-strength steel sheets

Mechanism of fatigue-failure in interstitial-free (IF-Ti) and interstitial-free high-strength (IFHS-Ti, Nb, B) steel sheets has been studied. In IF steel, the initiation and propagation of fatigue crack is largely through grain boundaries. This is attributed to the lesser cohesive strength of grain boundaries caused by the depletion of interstitials. In IFHS steel, fatigue cracks initiate primarily from microcracks within slip bands and crack propagation is largely transgranular. The reduced tendency of grain boundary cracking in IFHS steel is attributed to segregation of B and Nb to the grain boundaries.