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Author

Yankui Bian

Date of Award

1-2009

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Materials Science and Engineering

Supervisor

J.R. McDermid

Co-Supervisor

H.S. Zurob

Language

English

Abstract

TRIP-assisted steels combine high strength and good ductility, which makes them attractive to the automotive industry. Simultaneously, galvanizing is essential for the corrosion resistance of these steels. In industrial practice, the processing of TRIP-assisted steels and galvanizing process must be combined.

Two Al-alloyed TRIP-assisted steels (1.0Al: O.2C-1.SMn-O.SSi-1.0Al (wt.%) and 1.5Al: O.2C-1.SMn-1.SAl (wt.%)) were investigated in the present research work, where two points are noteworthy. First, the experimental processing routes in the present work are compatible with the continuous galvanizing process; second, it has been shown that the two steels exhibit good galvanizability. The initial microstructure and its evolution during plastic deformation of the two steels were examined. The kinetics of phase transformations taking place during thermal processing and plastic deformation were discussed. These results were linked to the work hardening behaviour with kinematic hardening taken into account.

It was confirmed that the retained austenite in the steels obtained through the present galvanizing heat treatments contributed to the work hardening behaviour by transforming to martensite during plastic deformation. The 1.SAl steel exhibited a better work hardening behaviour due to the more stable retained austenite.

Retained austenite of higher stability transformed to martensite more gradually during plastic deformation, efficiently attenuating decreases in the instantaneous work hardening rate, dσ/dε, and giving rise to a smoother evolution of the incremental work hardening coefficient, d(Lnσ)/d(Lnε). One of the attenuating mechanisms was the development of back stresses, which contributed kinematic hardening to the overall work hardening. Gradual transformation of retained austenite to martensite continuously supplied new obstacles to dislocations and delayed the saturation of back stresses.

Based on the present work and the previous work, it seems possible to process galvanizable Al-alloyed TRIP assisted steels using continuous galvanizing thermal cycles, which implies the possibility to combine continuous galvanizing and thermal processing of TRIP-assisted steels.

McMaster University Library

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