Abstract
	The aim of this work is to analyse the fatigue crack growth in structural steel with yield strength from 346 to 780 MPa with polyhedral ferrite-perlite, acicular and mixed ferrite-bainite-martensite microstructure. Influence of crack orientation to the rolling direction on fatigue results was observed, as well. Compact tensile CT test pieces were tested in fatigue at load factor R=0,05. For evaluation the test results have been fitted to the Pariso equation by calculation of the threshold stress intensity factor and coefficients C and m.
	Fractogrraphy of the fractured surfaces confirmed the transcrystalline evolution of the crack in both kinds of microstructure. The experimental results had shown that microalloyed steels with acicular or mixed microstructure even with high yield point values have good fatigue resistance. Threshold stress intensity factor ?Kp0 values ranged from 7,8 to 9,8 MPa.m1/2, the highest value obtained for the steel grade KODUR E 700. Influence of orientation of crack direction to the rolling direction is more distinct for steel with higher yield point.