Abstract
Pure martensitic steels have after hot forming limited performance in terms of rest ductility which limits the application in crash relevant parts. New steel grades were designed in the EU project HOTFORM including the corresponding process routes. These steel grades have ferritic-martensitic dual phase (DP) and martensitic-bainitic complex phase (CP) microstructures after hot forming process. The laboratory tests show an improved formability after hot forming. The basic concepts of the new alloys are explained. Furthermore, for validation of upscaling purposes a semi-industrial test is carried out and the results are discussed. The main application is for vehicle safety. This is evaluated by comparing the crash performance of these hot formed grades with cold rolled DP1000 and CP1000 for crash cans in a drop tower test.
Highlights
In automotive lightweight design, AHSS components are one of the preferred alternatives, since they allow downsizing vehicle structures in a cost effective manner
New steel grades were designed in the EU project HOTFORM including the corresponding process routes
The main application is for vehicle safety. This is evaluated by comparing the crash performance of these hot formed grades with cold rolled DP1000 and CP1000 for crash cans in a drop tower test
Summary
AHSS components are one of the preferred alternatives, since they allow downsizing vehicle structures in a cost effective manner. HOTFORM proposes a new route for manufacturing multiphase AHSS grades (DP type and CP type), where the annealing stage, from the steel processing, is performed at the press shop, heating the blank sheet in a furnace and hot forming the part, with cooled dies This will be achieved by optimized steel alloying design and dilatometry testing and characterization, aiming at ensuring the stability of the required phase transformation kinetics over the combined thermal and deformation gradients, produced during hot forming. For Complex-Phase alloys, CP1000 steel grade was the reference for designing alloys with a microstructure consisting of carbide-free bainite (> 90 %), martensite (< 10 %) and few amounts of retained austenite This microstructure provides high ductility while maintaining the strength. Based on dilatometer testing and flat tool testing for each steel type 1 composition was selected: DP-A and CP-B
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