Effect of cooling process after rolling on microstructure and property of Nb-Ti micro-alloyed low-carbon bainite steel

The influence of cooling process on microstructure, second phase precipitation and mechanical property of Nb-Ti Micro-alloyed Low-Carbon Bainitic Steel has been examined by metalloscope and TEM. The results lustrate that accelerated cooling after air-cooling (relaxation) to a certain temperature could obtain ferrite / bainite dual phase structure. The reduction of relaxation stop temperature results in increases of the volume of ferrite phase and grain size, and second phase size has trendy of grow up, the shape of bainite has change, while the bainite lath boundaries and the orientation become less obvious, dislocation density decreases, and the shape of M/A island changes, which leads to lower strength but improve yield ratio and ductility.