为满足用户定制的C250b门架型钢屈服强度不低于500 MPa，抗拉强度不低于610 MPa，断后伸长率不小于17%，-20℃冲击功不低于27 J的指标要求，设计了C-Mn-Nb-V-Cr-N体系钢，参考实际生产工艺，在不同的开轧温度和终轧温度下对试验钢进行精轧，研究了试验钢的组织和力学性能，分析了其强化机制。结果表明：设计试验钢的化学成分（质量分数/%）为0.16~0.20 C，0.40~0.50 Si，1.50~1.60 Mn，0.35~0.45 Cr，微量Nb+V，试验钢经精轧和空冷后的组织为铁素体+珠光体组织；当开轧温度控制在920~970℃，终轧温度控制在820~870℃时，试验钢的平均晶粒尺寸不大于7 μm，力学性能参数满足指标要求；试验钢固溶强化对屈服强度的贡献值约为240 MPa，细晶强化的贡献值为177~191 MPa，第二相析出强化的贡献值约为100 MPa。

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.