Stability of Retained Austenite in High-Al, Low-Si TRIP-Assisted Steels Processed via Continuous Galvanizing Heat Treatm
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TRODUCTION
LOW alloy transformation-induced plasticity (TRIP)–assisted steels typically contain ferrite, bainitic ferrite, high carbon (~1 to 2 wt pct) retained austenite (RA), and possibly a small amount of athermal martensite. It is usually desired that the RA gradually transform to martensite during plastic deformation, giving rise to the so-called TRIP effect. This deformation-induced transformation and the resultant composite microstructure of TRIP-assisted steels are traditionally believed to be primary contributors to the sustained high rates of work hardening exhibited by these steels, resulting in superior combinations of strength and ductility.[1] This combination of high strength and ductility makes TRIP-assisted steels attractive to the automotive industry as it permits the reduction of vehicle weight for better fuel efficiency while improving passenger safety. In particular, the superior high strain-rate performance of TRIP-assisted steels makes them strong candidates for safety-related J.R. McDERMID, Professor, and H.S. ZUROB, Associate Professor, are with the Steel Research Centre, McMaster University, Hamilton L8S 4L7, Canada. Contact e-mail: [email protected] Y. BIAN, formerly M.A.Sc. Student, Steel Research Centre, McMaster University, is now Research Engineer, with Evraz Inc. NA Research & Development Centre, Regina, SK S4P 3C7 Canada. Manuscript submitted November 24, 2010. Article published online April 6, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
applications in which high specific energy absorption is desirable.[2,3] The thermal processing of cold-rolled TRIP-assisted steels generally comprises two critical steps, namely, intercritical annealing (IA) and isothermal bainitic transformation (IBT) or austempering. The purpose of the IA is to form a microstructure of ferrite and austenite (c), which is then rapidly cooled to and held at the IBT for some time, during which the IA austenite transforms to bainitic ferrite and carbon enriched RA such that the martensite start (Ms) temperature of the RA is below room temperature. The precipitation of carbides from austenite during the IBT is typically delayed through alloying additions such as Si[4] and/or Al.[5–8] It is the objective of the IBT to form sufficient amounts of high-carbon austenite (typically 10 to 15 vol pct) such that the austenite gradually transforms to martensite during deformation in order to produce high sustained work hardening rates, thereby delaying the onset of necking and producing an alloy with superior combinations of strength and ductility. If the steel is to be used for exposed automotive structures, it is also necessary to protect the alloy against corrosion, for which the continuous galvanizing (CGL) process is amongst the most cost effective for the production of large tonnages of sheet steel products. However, there are some fundamental difficulties which must be addressed in the production of TRIP-assisted steels by the CGL process. The primary issue which needs to be addressed when attempting to galvanize VOLUME 42
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