Abstract:Effect of air-cooling relaxation temperature during the controlled cooling on the microstructure and mechanical properties of hot-rolled Nb microalloyed steel was investigated. The results showed that the multiphase microstructure containing ferrite, bainite and retained austenite can be obtained by relaxation and ultra-fast cooling after rolling, while the final temperature of relaxation significantly influences the ferrite grain size, ferrite content, dislocation density and precipitation amount on the ferrite matrix. The yield and tensile strengths decrease and the yield ratio approximately remains the low level of 0.7 with the reduction of relaxation stop temperature. When the final temperature of relaxation is 720℃, tensile strength, total elongation and product of tensile strength and ductility reach the maximum values, ie., 820MPa, 37.5% and 30750 MPa•%, respectively.