Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure
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INTRODUCTION
NUMEROUS studies since the 1970s have shown that dual-phase steels can present an improved balance of strength and ductility in comparison to the low-carbon steels commonly used in the automotive industry. This improvement is due to the combination of a ductile ferrite matrix with a dispersion of hard martensitic grains. This microstructure is obtained after intercritical annealing, during which a controlled volume fraction (;20 pct) of austenite is formed. This austenite is subsequently transformed into martensite by water quenching.[1,2,3] Dual-phase steels always contain small amounts of retained austenite at room temperature.[4,5] This austenite was usually considered as a minor phase of the microstructure. Nevertheless, some studies have theoretically demonstrated that retained austenite can improve the mechanical properties of dual-phase steels through the transformation-induced plasticity (TRIP) effect, especially if the stability of retained austenite can be increased.[6,7,8] In more recent years, new formable high-strength steels have been obtained by generating more complex multiphased microstructures, in which retained austenite can bring about a more efficient TRIP effect. These so-called ‘‘TRIPassisted multiphase steels’’ are obtained when the intercritical annealing stage is followed by an isothermal tempering stage in the bainite transformation temperature range.[9,10,11] During this tempering, part of the austenite transforms to bainite, whereas a large amount of residual austenite may P. JACQUES, Graduate Student, and F. DELANNAY, Professor, are with the De´partement des Sciences des Mate´riaux et des Proce´de´s, Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium. X. CORNET, Research Engineer, and Ph. HARLET, Engineer and Head of Products Research Department, are with the Research and Development Center, Cockerill Sambre Group, B-4000 Lie`ge, Belgium. J. LADRIERE, Professor, is with the Laboratoire de Chimie Inorganique et Nucle´aire, Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium. Manuscript submitted September 30, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
become sufficiently stabilized by carbon rejection from bainite as to not transform to martensite during the final water quench. Indeed, it is well known that a significant amount austenite can be retained in bainitically transformed steels highly alloyed with silicon (;2 wt pct).[12] Austenite transformation then occurs during straining, as in the fully austenitic TRIP steels.[13,14,15] Hence, retained austenite is a major phase of TRIP-aided steels, and the TRIP effect is believed to be the main phenomenon which explains the high strength–high ductility balance exhibited by these steels. As manganese and silicon have a beneficial contribution to the retention of austenite, conventional TRIP-assisted steels always contain large concentrations of these elements (from 1.5 to 2.5 wt pct for both). Manganese is known to be an austenite stabilizer. Silicon is reported to stabilize austeni
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