To satisfy the demand for high-strength steel sheets with excellent strength–ductility–hole expansion ratio balance, high-strength steels consisting of a ferrite or bainitic ferrite matrix strengthened by a large amount of finely dispersed precipitates have been developed.
In this study, the influence of the precipitation-forming elements Ti and Nb on the strength–ductility–hole expansion ratio balance of these steels has been investigated. The strength–ductility–hole expansion ratio balance of steel with Ti added was superior to that with Nb added. The reason for the superiority of steel with added Ti was discussed from various viewpoints such as microstructure, texture, the condition of the pierced surface, sulphide formation, the nucleation and propagation behaviour of cracks during hole expansion, etc. We concluded that the inferior strength–ductility–hole expansion ratio balance of the steel with added Nb was mainly caused by the formation of large textural colonies and a detrimental influence of MnS. From these inferences, we proposed a combined addition of Ti and Nb, and succeeded in improving the strength–ductility–hole expansion ratio balance of the advanced high-strength steel.