Materials Transactions Online

Materials Transactions, Vol.46 No.08 (2005) pp.1839-1846
© 2005 The Japan Institute of Metals

The Role of Retained Austenite on Tensile Properties of Steels with Bainitic Microstructures

Carlos García-Mateo and Francisca G. Caballero

Department of Physical Metallurgy. Materalia Research Group. Centro Nacional de Investigaciones Metalúrgicas (CENIM-CSIC), Avda. Gregorio del Amo, 8, 28040, Madrid, Spain

In high-carbon, silicon-rich steels it is possible to obtain a very fine bainitic microstructure by transformation at low temperatures (200--300°C). This microstructure consists of slender ferrite plates, with thicknesses of several tens of nm, in a matrix of retained austenite. Whereas strength is mainly provided by to the fine scale of the ferrite plates (stronger phase), ductility is mostly controlled by the retained austenite (softer phase). Further improvement in ductility is achieved by strain induced transformation of austenite to martensite, the so called TRIP effect. In order to take full advantage of this effect, the mechanical stability of the austenite, i.e., its capability to transform to martensite under strain, must not be too low nor excessively high.
Two main aspects of the mechanical stability of the retained austenite, morphology and chemical composition, have been studied to determine the role that these play on the ductility behaviour of the bainitic steels studied. It is suggested that the chemical composition has the strongest effect on the ductility of these new high strength alloys.

(Received 2005/3/1; Accepted 2005/6/8; Published 2005/8/15)

Keywords: Bainite, retained austenite, mechanical stability, morphology, chemical composition, ductility, strength

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