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Abstract

The main targets of modern steel development for car bodies are light weight and safety. Multi-phase high-strength steels, e.g. DP (dual phase) and TRIP (transformation induced plasticity) steels, have been developed in order to meet these targets. These steels offer the option of bake-hardening properties which have not been thoroughly exploited so far. Thus, the aim of this European project was to investigate the fundamentals of the bakehardening behaviour of modern multi-phase steels and to evaluate the dominant process and microstructural parameters. The yield strength increase during paint baking in conventional BH steels with ferritic microstructure is caused by the Cottrell effect of decoration of dislocations with interstitial atoms and by precipitation of carbides. In multi-phase steels, additional microstructural effects contribute to the strength increase during baking treatment. In DP steels, an internal stress relief takes place at the ferritemartensite interfaces. In TRIP steels, it has to be taken into account that the microstructure changes during prestraining and that the stability of the phases changes during baking treatment. Thus carbon partitioning into the retained austenite phase could release internal stresses while a stress relief in bainite may be responsible for a strength increase. In general, it can be stated that both DP and TRIP steels show significant yield strength increases during baking treatment, which, occurs under some circumstances, can be even more pronounced than in conventional BH steels. The obtained scientific results obtained can be used to optimise microstructures and process routes for multi-phase steels with reliable bake-hardening properties.

Additional information

Authors: BLECK W, RWTH Aachen University, Aachen (DE);BRÜHL S, RWTH Aachen University, Aachen (DE);GERBER T, ThyssenKrupp Stahl AG, Dortmund (DE);KATSAMAS A, Laboratory of Materials, University of Thessaly, Volos (GR);OTTAVIANI R, Arcelor Research SA, Maizières-lès-Metz (FR);SAMEK L, Laboratory for Iron and Steelmaking, RUG, Gent (BE);TRAINT S, Division Stahl, VoestAlpine Stahl Linz, Linz (AT)
Bibliographic Reference: EUR 22448 EN (2007), 169 pp. Euro: 25
Availability: Katalogue Number: KI-NA-22448-EN-S The paper version can be ordered online and the PDF version downloaded at: http://bookshop.europa.eu
ISBN: ISBN: 978-92-79-04608-7
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