Effect of Cooling Process After Rolling on Microstructure and Mechanical Property of Nb-Ti Microalloyed Low-Carbon Bainite Steel

The effects of cooling process after rolling on microstructure,precipitated phase and mechanical properties of low-carbon bainite steel were investigated by method of SEM and TEM.The results show that ferrite and bainite dual phases can be obtained with air-cooling and accelerated cooling process after hot-rolled.The percentage of ferrite phase increases with the end of air-cooling temperature reduced,the size of ferrite and precipitated phase is larger,the lath of bainite is sharper and the edge is clearer.The shape of M/A constituent is film-like and changes with temperature.When temperature of completion in air-cooling are between 725 ℃ and 740℃,then it is cooled to 440 ℃ with cooling rate of 15 ℃/s,the comprehensive mechanical properties are excellent and can reach the requirements of Q690 of GB/T1591.