Abstract: Low C-Nb-V composite microalloyed steel was designed to meet the requirements of strength upgrading and efficient welding of hull steel. The microstructure and mechanical properties of 440 MPa steel at different final rolling temperatures(FRT)and different final cooling temperatures(FCT)were studied.The results show that during final rolling in the unrecrystallized zone, with the decrease of FRT, the grains are refined gradually, and rolling in critical zone greatly increases the density of low and high angular grain boundaries(LAGB, HAGB)in the steel.In the mean time, the pearlite band degrades gradually, and the microstructure change from ferrite+pearlite to qusi-polygonal ferrite+a little pearlite. The strength of the steel increases gradually while -40 ℃impact energy remains stable. With the decrease of FCT, the refinement of grains is inconspicuous, but the density of LAGB, HAGB increases significantly.The microstructure of the steels changes from ferrite+pearlite to qusi-polygonal ferrite+pearlite and then to bainite+acicular ferrite. Both strength and -40 ℃impact energy of the steel are improved.