Abstract
The prevailing societal pressures are guiding the car industry to deliver vehicles with improved safety, reduced environmental impact and extended service life. One way towards these targets has been the reduction of fuel consumption by lowering the car weight. This favours the use of ultra high strength steels, which enable reduced sheet thicknesses. Thin gauge and long service life requires reliable corrosion protection. Thus, hot dip galvanised dual phase steels are an interesting option in automotive applications. The microstructure of these steels consists of a dispersion of hard martensite islands in a soft ferrite matrix. Due to their microstructure the dual phase steels offer a good combination of strength and ductility. The low level of alloying elements provides good weldability.
The current study has concentrated on mechanical properties, high strain rate behaviour and formability as well as spot weldability of hot dip galvanised ultra high strength dual phase steels. A family of continuously hot dip galvanised commercial dual phase steels with tensile strengths ranging from 600 to 1000 MPa were used in the experiments. The microstructure, mechanical properties and formability of sheet materials were determined. The high strain rate behaviour was determined with as-shipped and in selected cases with bake hardened specimens by dynamic tensile tests. Spot welding lobes were determined.
The present results show that in addition to applications where good formability is needed, the hot dip galvanised ultra high strength steels can be used in applications where energy absorption potential at high strain rates is needed. As the strain rate increased both yield and tensile strength increased. Energy absorption behaviour was comparable with that of cold rolled DP steels. The spot welding lobes were large for the investigated grades and acceptable welds could be produced using relatively low welding currents.