STUDY ON STRENGTHENING MECHANISM OF 650 MPa GRADE V-N MICROALLOYED AUTOMOBILE BEAM STEEL

Abstract

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 increas- ing 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 mecha- nism and contributes 43.05% to the yield strength. And the contribution of precipitation strengthening and disloca- tion strengthening to the yield strength is up to 34.44%.