Abstract: By high cycle fatigue test on MTS-810 material testing system, the fatigue property of new developed hot-rolled Nb and Ti microalloying 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are studied, and the effects of grain size, second phase (precipitation and inclusion), high/low angle grain boundaries and carbide on fatigue property are investigated. The result indicate that, when the load ratio R of –1 and cycle base number of 107, the conditions fatigue strength of 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are 438 MPa and 443 MPa, respectively. The fatigue strength ratio of 700 MPa grade carriage strip and 780 MPa grade crossbeam strip are 0.61 and 0.57, respectively, which higher than general steel. Fatigue crack source of experimental steel are persistent slip bands(PSB) and coarse precipitation or inclusion. When the size of precipitation is small, the critical fracture stress of crack instability propagation is higher, so nano scale precipitation very hard to become fatigue crack source. When the size of second phase is nearly, contrast to rounded closed angle coarse precipitation or inclusion, the quadrate precipitation or inclusion are easily to generate stress concentration and become to fatigue crack source. The reasons of experimental steels having excellent anti-fatigue performance are ultra fine grain size, nanometer precipitation and granular or short bar carbides.

Key words: Fatigue, Fatigue strength ratio, Microalloy steel, Second phase, Ultra-high strength steel