Stamping and Welding Experience with Ultra High Strength Steels for Automotive Body Applications

One of the possibilities to fulfill the enhanced requirements on crash performance for new vehicles is the application of high strength steels (HSS) and even ultra high strength steels (UHSS).

In order to achieve the strength, the strain rate sensitivity can be taken into account whereas the work hardening effect is difficult to be used. For UHSS with more than 500 MPa yield point the formability, spring back and weldability are important issues. In the present study the laser weldability of UHSS has been studied with positive results. Both Dual phase type and micro alloyed type LWBs showed high formability and good weldability and will satisfy the requirement of certain applications for body members. Stamping trials in the press shop under production conditions showed that the peak strain in certain parts can be reduced to avoid splits when LWBs were stamped. Simultaneously the spring back is reduced as well.

We can conclude that for a suitable design of parts, UHSS can be used for BIW applications. From the stamping point of view in the press shop, the dual phase type steels are more favorable concerning the avoidance of splitting. The spring back of UHSS parts is a problem. However, micro alloyed steels showed lower spring back than the investigated dual phase steels. The micro hardness of spot welds and laser seam is higher than usual. The fatigue properties of parts and sub-assemblies will be the next subject to be investigated.