Fast warm stamping of ultra-high strength steel sheets

High strength steels are uniquely lightweight and engineered to meet the challenges of today’s vehicles for stringent safety regulations, emissions reduction, solid performance, at affordable costs. To deploy high strength steel in mass scale automotive manufacturing, an effective and scalable sheet metal forming technique is required. Processes using hot or warm stamping are emerging as solutions for forming high-strength parts from steel sheets, e.g. for applications in automotive “body in white” (BiW), and chassis and suspension (C&S) parts. The global demand for such ultra high-strength steel parts has been growing sharply in recent years.

Existing methods for forming high strength steels involves heat treating the steel sheet to be formed at sufficiently high temperatures (e.g. more than 900 ˚C) for a prolonged period of time. Austenitisation is thus enabled, thereby prompting a phase change to a softer phase of the material. This aspect of the existing methods is energy intensive, and is known to take approximately 75% of the overall processing time to create the finished part. Furthermore, using existing methods, the hot stamped part is held in cold dies, with a high cooling rate (e.g. more than 25 °Cs-1 on average) to enable the hardest phase of the material to be formed. This too is time consuming.

Leading researchers from Imperial College have devised a fast warm forming method that overcomes these shortcomings to improve manufacturing productivity of high-strength metal sheet parts. Using this novel method, cycle times and energy costs are reduced, whilst avoiding major negative changes to the microstructure, retaining a similar strength post-forming vs pre-forming. Current hot stamping methods could take 12-16 s per forming cycle (without considering time for heating the blank), whilst our technology could take just ~6 s including heating, forming and quenching.