Material design of 690-780 MPa grade hot-rolled sheet steels for wheel use

Hot-rolled sheet steels of a low Si composition strengthened by the addition of Ti and Nb have been investigated to develop 690 to 780 MPa grade materials for wheel disks and rims. The hot rolled sheet steels of 0.09%C-0.06%Si-1.5%Mn base composition with various concentrations of Ti and Nb were prepared in the laboratory under various post-rolling cooling conditions. Then the influence of microstructures and microalloying of Ti and Nb on mechanical properties were investigated. It has been found that the steels with a mixed structure consisting of ferrite and bainite show superior hole expandability, while the steels with ferrite and pearlite microstructures have better elongation. In these steels, the addition of Ti and Nb effectively increased their tensile strength, and the strength over 700 MPa was attained when 0.1% of Ti and 0.04% of Nb were alloyed with them. Based on the results, 690 to 780 MPa grade hot-rolled sheet steels for wheel disks and rims were manufactured on trial using a Ti- and Nb-added low-C low-Si composition and controlling their microstructures by post-rolling cooling. The developed steels successfully met the requirements for wheel disks and rims. (AU)