Study of TRIP-Aided Bainitic Ferritic Steels Produced by Hot Press Forming
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TRODUCTION
HIGH strength steel grades have been increasingly applied to automobile bodies to improve crashworthiness for occupant safety and improve fuel economy by reducing vehicle weight. Hot press forming technology is increasingly being applied to produce automotive structural members such as a B-pillar with ultra high strength in excess of ~1300 MPa. A typical steel used in this process is 22MnB5, which is formed in the fully austenitic state followed by die-quenching to generate a fully martensite microstructure that provides the high strength. The ductility, however, is generally a maximum of 6 pct,[1,2] but is considered adequate when components are designed for minimum intrusion into the passenger compartment. TRIP-induced carbide-free bainitic ferrite (TBF) steel has been reported to produce higher ductility and also maintain high strength compared with only high strength provided by a fully martensitic matrix.[3–6] TBF steel is characterized by a multi-phase microstructure, consisting of carbide-free bainitic ferrite matrix, and a mixture of carbon enriched retained austenite and some martensite. The formation of this microstructure is SHANGPING CHEN, Principal Scientist, and CHRIS LAHAIJE, Manager Automotive Technology Projects, are with Tata Steel, 1970 CA IJmuiden, The Netherlands. Contact e-mail: Shangping.Chen@ tatasteel.com RADHAKANTA RANA, Principal Researcher, formerly with Tata Steel, is now with Department of Metallurgical & Materials Engineering, Advanced Steel Processing & Products Research Center, Colorado School of Mines, Golden, CO 80401. Manuscript submitted August 1, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
due to the fact that the precipitation of cementite during bainitic transformation is suppressed by alloying the steel with sufficient Si and/or Al which have very low solubility in cementite, to greatly retard growth of cementite from austenite. The carbon that is rejected from the bainitic ferrite enriches the residual austenite, thereby increasing its stability. The austenite either transforms to martensite during cooling after the bainitic transformation or remains untransformed down to room temperature depending on its stability. The carbide-free bainitic ferrite in TBF steel is present in the form of plates with an ultra fine grain size (typically ~5 lm long and ~200 nm thick) and is the main contribution for the improved strength and ductility. The retained austenite provides the TRIP effect, and is beneficial for improving total elongation. The presence of martensite islands leads to high strength and to high instantaneous hardening rates. If the synergistic effect between TRIP attained by residual austenite and use of fine plate bainitic ferrite in TBF steel is maintained, a significant improvement of elongation of TBF steel can be attained.[3,4] The objective of this work is to study the possibility of developing high strength ductile steels by applying the hot press forming process, which would serve as a better alternative to traditional press hardened steels. This has bee
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