Development of Ultra-Fine Precipitation Hardened Hot-Rolled High Strength Steel and Its Application to Automotive Use

Environmental issues require auto makers to improve the fuel consumption of motor vehicles. One way of saving energy is to reduce the total weight of the vehicle by using high strength steels. However, with conventional steels, increasing tensile strength (TS) reduces elongation and stretch flange formability. JFE Steel developed a new TS/780MPa grade of hot-rolled high strength steel with excellent stretch flange formability in which the ferrite matrix is strengthened by ultra-fine, nano-sized precipitates. Transmission electron microscopy (TEM) reveled that the precipitates in ferrite grains are TiMoC₂ and their average diameter is less than 10 nm. The steel has an extremely high YS/TS ratio and an excellent balance of elongation and the hole expansion ratio, superior to that of TS/780MPa grade dual phase and HSLA steels. Moreover, because of its Si-free alloy chemistry, the new steel has good coatability and can be used as a substrate for galvanized or galvannealed steel sheets. The developed steel is expected to be widely used in automobile chassis and structural parts and has been already applied, for example, to suspension lower arms.