Effect of alloying elements on microstructure and mechanical properties of hot rolled multiphase steels

The compositional development and thermomechanical processing of multiphase steels - high strength microalloyed steels with good formability - are reviewed. Optimisation of austenite grain size with titanium or Ti + B additions combined with appropriate controlled rolling and cooling during runout and coiling allow martensitic steels with tensile strengths > 1200 MPa to be obtained. Silicon additions have been found to reduce sensitivity to variations in coiling temperature and hence scatter in mechanical properties. These steels have found application in safety critical automotive components where their higher strength can allow the use of thinner, lighter sections. Martensitic grades can reduce production costs by eliminating the need for an additional heat treatment step. The influence of niobium and titanium on the properties of dual phase and TRIP steels are also discussed.