Niobium in microalloyed engineering steels, wire rods and case carburized products

Microalloying technology, developed for the production of flat products (plate, strip and line pipe) during the 1960’s and 1970’s, has been applied to “long products” such as engineering bars, sections, forgings and wire rod since about 1980. In the 1980’s the main rationale to use niobium bearing steel bars and wires was to eliminate the need for a hardening process, i.e. quenching and tempering, in manufacturing heat-treated steel parts without any trade off in properties. Owing to the significant cost reductions and energy savings by adopting microalloyed steels, they have replaced conventionally heat-treated steels for connecting rods, suspension components and fasteners etc., in the automotive industry. On the other hand, the effect of Nb on the mechanical properties of heat-treated parts has also been clarified, and new steel grades, e.g. high strength steels for fasteners with 1500 MPa tensile strength and spring steels used in 1200 MPa design stress applications, have been developed. Furthermore, the immense capacity of niobium precipitates in steel to prevent grain coarsening during heat treatment has been utilized in developing case hardening steels for transmission gears and CVJ shafts, which are cold- forged and then carburized at a high temperature. The new grades have also succeeded in eliminating softening treatments such as spheroidized annealing and/or bright normalizing treatment in the manufacturing sequence. (AU)