Microstructure evolution of 400 MPa class rebar produced by QST and VNM technology under the high strain and low cycle fatigue

Abstract

The 400 MPa grade hot rolled rebar are widely used as the construction and building materials. The main produced technologies for this rebar are V-N Microalloyed (VNM) and Quenched and Self-Tempered (QST). Under the high strain and low cycle fatigue (HSLCF) loading, the microstructure evolution and performance of above two types of rebars were investigated systematically. In order to evaluate the rate of energy-absorption under HSLCF, a new model named cyclic toughness was proposed. The results show that QST rebar exhibit greater rate of cyclic softening over ±1% strain amplitude, which caused by the secondary cracks in the quenched and self-tempered martensite microstructure of surface layer and the formation of larger dislocation cell structure. Additionally, VNM rebar had higher cyclic toughness (15.4 J/cm3) due to higher dislocation density and smaller dislocation cell structure in the cyclic hardening stage, which indicate that the HSLCF performance of VNM rebar are much better than QST rebar.