Abstract: Mn-Nb-V-Cr-N series steel was designed in order to meet the requirements of C250b gantry steel customized by users with yield strength of not less than 500 MPa, tensile strength of not less than 610 MPa, percentage elongation after fracture of not less than 17%, and impact energy at -20 ℃ of not less than 27 J. According to the actual production process, the test steel was finishing rolled at different initial rolling temperatures and end rolling temperatures. The microstructure and mechanical properties of the test steel were studied, and the strengthening mechanism of the test steel was discussed. The results show that the designed steel had the chemical composition (mass fraction/%) of 0.16-0.20 C, 0.40-0.50 Si, 1.50-1.60 Mn, 0.35-0.45 Cr, trace Ni and V, and the microstructure of the steel was ferrite + pearlite after finishing rolling and air cooling. When the initial rolling temperature was 920-970 ℃ and the end rolling temperature was 820-870 ℃, the average grain size of the test steel was no greater than 7 μm; the mechanical property parameters of the test steel met the requirements. The contribution of solution strengthening in the test steel to the yield strength was about 240 MPa, that of fine-grain strengthening was 177-191 MPa, and that of precipitation strengthening of the second phase was about 100 MPa.