Abstract: In order to improve the local formability of 980 MPa high-strength steel, the microstructure and mechanical properties of 980 MPa high-strength steel with different annealing process paths were studied by means of universal testing machine, field emission scanning electron microscope(SEM), electron backscattering diffraction(EBSD) and comprehensive forming testing machine. Moreover, its effect on hole expansion ratio and local formability were evaluated. The results show that in addition to the two phases of ferrite and martensite, the microstructure of the new quenching & tempering(Q&T) process also contains tempered martensite, the average grain size of ferrite and martensitic is 3.14 μm and 2.62 μm, respectively, and the martensite area fraction is 61.0%, while the typical ferrite and martensitic dual phases are obtained under the traditional process, the average grain size of ferrite and martensite is 4.77 μm and 2.77 μm, respectively, and the martensite area fraction is 35.8%. The elongation of the two processes is comparable, but the yield strength and hole expansion ratio are significantly different, and the higher yield ratio and hole expansion ratio are achieved under the new Q&T process, which is attributed to its smaller ferrite grain size and the difference in ferrite and martensite hardness. The ratio of true fracture strain(TFS) to true uniform strain(ε_u) of the traditional process is 7.0, while the new Q&T process is 15.2, therefore the new Q&T process has excellent characteristics in local formability.

Key words: 980 MPa grade high-strength steel, microstructure, hole expansion, local formability