Research on Microstructure and Mechanical Properties of Ti+Nb and Ti+V Ultra-low Carbon Bake Hardening Steels

Deformation resistance and dynamic continuous cooling transformation of Ti+Nb and Ti+V bearing ultra-low carbon bake hardening steels were studied using Gleeble-1500 thermal simulator.The microstructures of two experimental steels under different cooling conditions were also analyzed.The results showed that the deformation resistance of Ti+Nb bearing ultra-low carbon bake hardening steel was 13MPa higher than that of Ti+V bearing ultra-low carbon bake hardening steel at 1100℃ with the strain rate of 1s-1.The morphology of the microstructures and the grain size have big difference for two experimental steels under the same deformation conditions.The room temperature microstructures of the two experimental steels are polygonal ferrites under various cooling conditions.The ferrites of Ti+Nb bearing ultra-low carbon bake hardening steel are finer and have irregular shape with the average grain size of 16μm,while for Ti+V bearing ultra-low carbon bake hardening steel,the ferrites are coarse and have regular shape with the average grain size of 26μm.