Automobile beam steel with high strength is the development trend of the automotive industry. With the development of heavy-duty vehicles, low cost automobile beam steel both with high strength and high toughness need to be developed. V-N microalloying method combined with thermal mechanical controlled process has a significant grain refinement function. Therefore, the relationship of different N content and technology should be studied in detail. In this work, the microstructure and precipitates of V and V-N microalloyed steel were investigated by using OM, SEM and TEM. And their strengthening mechanism was studied. The results show that both V microalloyed steel and V-N microalloyed steel mainly consist of ferrite and little pearlite. With the increasing of coiling temperature, the strength increased first and then decreased. The optimum mechanical properties were obtained when coiling at 600 ℃, the yield strength, tensile strength and elongation reached 605 MPa, 687 MPa and 24.5%, respectively. Compared with V microalloyed steel, the ferrite in V-N microalloyed steel present finer grain size which can be refined to about 4.5 mm. The precipitates were finer and more dispersed which distribute mainly between 3~50 nm and have the average size of 8.0 nm. And the dislocation density in V-N microalloyed steel is higher. Ferrite grain refinement strengthening, precipitation strengthening and dislocation strengthening make V-N microalloyed steel possess higher yield strength. Ferrite grain refinement strengthening is the predominant mechanism and contributes 43.05% to the yield strength. And the contribution of precipitation strengthening and dislocation strengthening to the yield strength is up to 34.44%.