Current research shows that several materials with application in the automotive, railway, aeronautical and aerospace industries do not have a fatigue limit. However, around 1860 Wohler proposed the S-N curve (stress - number of cycles), admitting a concept that there is a fatigue limit, stipulated at the time as 1E5 cycles and considered as infinite fatigue life. Although, with technological advances, numerous components fail in the condition of infinite life, contrary to what Wöhler proposed. Currently, 3 different fatigue regimes are considered: low cycle fatigue, high cycle fatigue and very high cycle fatigue. In the automotive industries there is an extensive use of sheet steel for the manufacture of components with complex geometry. In this context, interstitial free steel (IF steel) is one of the widely used materials, due to its excellent stampability. IF steel has a ferritic microstructure with a very low amount of interstitial elements. Interstices with carbon and nitrogen are stabilized by titanium (Ti) or niobium (Nb) forming extremely small volume fractions in the ferritic matrix of carbides and nitrides. The work is based on the experimental investigation of microstructural damage in IF steel after long-term cyclic loading. Samples of Ti-stabilized IF steel were subjected to ultrasonic fatigue tests with loads greater than 1E7 cycles. It was observed the arising of microstructural damage (persistent slip bands, PSB s) after a certain number of cycles. By means of image analysis in the optical microscope, it was estimated that 97.76 percent of the fatigue life was destined for crack nucleation while 2.23 percent for crack propagation in very high cycle fatigue regime. There was the project of a new specimen for the very high fatigue machine, in order to accompany the surface of the material during the tests. It is proposed the equation of the curve S-N fatigue of very high cycle os with data extracted from the tests. Finally, a correction factor is suggested in the plastic shear deformation equation in PSB s based on experimental data.