Digital Repository, CONVEGNO IGF XXII ROMA 2013

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Fatigue endurance of new high-strength car-body steels
P. Matteis, G. Scavino, D. Firrao, R. Sesana, F. D’Aiuto

Last modified: 2013-06-27


Advanced, high-strength steel sheets are increasingly used to make lighter and safer car-bodies. The
main mechanical requirements of these steels in service are high fracture strengths and energy absorptions,
relevant for crash tests, and fatigue endurances, relevant for ordinary car usage. The steel sheets are made by
continuous casting, hot rolling, cold rolling and continuous annealing, or other continuous final heat treatments,
and are then cold formed and welded to fabricate the car bodies. Two new high-strength car-body steels are
examined here: a TWIP (TWinning Induced Plasticity) steel, which is already industrially available, but not yet
widely used, and a Q&P (Quenching and Partitioning) steel, which was recently produced industrially as a
prototype. The new steels are compared with a widely used, high-strength, DP (Dual-Phase) automotive steel
grade. Automotive TWIP steels are high-Mn austenitic steels, with a medium-high C content, which exhibit a
promising combination of strength and toughness, arising from the ductile austenitic structure, which is
strengthened by C, and from the TWIP (TWinning Induced Plasticity) effect. The low-alloy Q&P steels are
subjected to the Quenching and Partitioning (Q&P) final heat treatment, which consists of: 1) full or partial
austenitizing; 2) quenching to the Tq temperature, comprised between Ms and Mf; 3) soaking at the Tp
“partitioning” temperature, equal to or slightly higher than Tq, allowing carbon to diffuse from martensite to
retained austenitized; 4) quenching to room temperature. The final microstructure consists primarily of lowcarbon
martensite, high carbon martensite, and carbon stabilized austenite. The fatigue behavior of these steels
is examined both in the as-fabricated condition and after pre-straining and welding operations, which are
representative of the cold forming and assembling operations performed to fabricate the car-bodies. Moreover,
the microscopic fracture mechanisms are assessed by means of fractographic examinations.

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