The tensile strength levels required for spring and gear applications are far higher than those commonly associated with other uses of microalloyed steels. In addition, springs and gears are manufactured using highly specific processes. This is why we present first of all a brief overview of the context and objectives of these developments, in order to assess their scope. The work presented here confirms that the effects of Niobium are not simply those usually associated with the precipitation of Nb(CN) carbonitrides: inhibition of recrystallisation, control of austenitic grain coarsening and precipitation hardening . Account must also be taken of the interactions between the Nb and other elements, as well as the slowing down of g => a + carbides transformation observed in certain cases. The number of mechanisms (or interactions) identified clearly shows why there is still a broad range of choices for optimising steel products and processes according to the target application, the production characteristics and the desired value for money ratios.