Abstract: Based on the principle of automobile lightweight, a 1000 MPa hot-rolled dual phase steel was prepared by hot rolling, ultra fast cooling and relaxation heat treatment, and effect of relaxation time on microstructure and mechanical properties of the tested steel was studied. The results show that with the increase of relaxation time, the banded distribution of ferrite and martensite of the tested steel is more obvious, the ferrite grain size and volume fraction increase, the yield strength decreases and the elongation increases. The tensile strength first increases and then decreases, which is the result of the combined action of martensite volume fraction and carbon content. The yield ratio decreases, and the n value increases. The relaxation time affects the volume fraction, grain size and internal substructure of dual phase. When the relaxation time is 10 s, tensile strength, elongation, yield ratio and n value of the tested steel are 1025 MPa, 17.5%, 0.48 and 0.13, respectively, under which shows the best comprehensive properties. Considering the mechanical properties and production efficiency, the appropriate relaxation time of the tested steel under the process conditions is 7-10 s.

Key words: hot-rolled dual phase steel, relaxation heat treatment, microstructure, mechanical properties