Abstract: To study the microstructure and properties of Q345 grade bridge hot-rolled H-section steel in the Nb-Ti system,two alloy composition designs, namely high niobium with low titanium and high titanium with low niobium, were adopted for experimental trials. The experimental results show that the mechanical properties of the tested steels from both the high niobium and low titanium alloy system and the high titanium and low niobium alloy system meet the requirements of the Q345q steel grade(yield strength not less than 345 MPa) D and E level(impact energy no less than 120 J at -20 and -40 ℃). Among them,the high titanium and low niobium composition has excellent strength indicators,but the impact toughness is slightly lower. Through microstructure analysis,the microstructure of the high niobium and low titanium composition is mainly ferrite+pearlite in the room temperature,while that of the high titanium and low niobium composition is mainly bainite+ferrite+minor pearlite. By using SEM scanning electron microscopy for observation,it was found that the C atom content in the pearlite region of the high-niobium and low-titanium composition test steel was slightly higher than that in the ferrite region. However,for test steel B with high titanium and low niobium,the C atom content in the pearlite region was not significantly different from that in the ferrite region. This indicates that a metastable phase transformation occurred in the microstructure of test steel B,eventually forming a bainite structure enriched with carbides, thereby enhancing the strength of the product.