Application of solute drag theory to model ferrite formation in multiphase steels
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NTRODUCTION
DURING recent years, advanced high-strength steels have been proposed, which offer significantly improved property combinations as compared to conventional high-strength low-alloy steels. These novel, low-carbon steels are multiphase steels including dual-phase (DP), transformationinduced–plasticity (TRIP), and complex-phase (CP) steels, which combine high strength with excellent formability. This development is particularly stimulated by the demands of the automotive sector, which is increasingly required to build lightweight, fuel-efficient vehicles. For example, DP steels are projected to become a key component in new automobile designs; i.e., to account for 74 pct by weight of the proposed vehicle body structure.[1] As a result, significant research effort is being devoted worldwide to develop new generations of multiphase steels.[2,3,4] The addition of alloying elements such as Mn, Si, Al, Cr, Mo, etc., is crucial for these steels, either for principal metallurgical reasons as in TRIP steels or for practical considerations of the industrial production process. In particular, alloying additions are exploited to control the austenite ()–to–ferrite () transformation, thereby obtaining the desired multiphase microstructures. Thus, there is also a renewed interest in understanding the effects of alloying elements on the transformation kinetics from a more fundamental point of view. In this regard, the austenite decomposition into ferrite is of particular significance since, as a precursor of the formation of nonequilibrium structures; i.e., F. FAZELI, Postdoctoral Fellow, and M. MILITZER, Professor and Dofasco Chair in Advanced Steel Processing, are with the Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted January 27, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
pearlite, bainite, and martensite, a specified fraction of ferrite must be present. In spite of the extensive studies on the ferrite formation kinetics that have been conducted since the 1930s, there is still a surprising lack of illumination of the exact role which substitutional elements play in this scenario. Presumably, this can be attributed to their complex behavior during ferrite growth, which is related to the degree of their partitioning between parent and product phases, and also their potential drag effect on the moving / interfaces due to solute segregation. Austenite decomposition into ferrite is a diffusional nucleation and growth process, in which both of these phenomena contribute to the overall reaction kinetics. However, for industrial processing routes, the transformation kinetics is usually solely dictated by ferrite growth, since either the transformation starts from an austenite-ferrite microstructure or the imposed cooling paths are such that nucleationsite saturation is obtained in the initial transformation stages. The growth kinetics of ferrite in Fe-C alloys has been frequently described b
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