Retained Austenite Stabilization Through Solute Partitioning During Intercritical Annealing in C-, Mn-, Al-, Si-, and Cr
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-Mn transformation-induced plasticity (TRIP) steels are one of the promising ‘‘Third Generation’’ advanced high-strength candidate steels (AHSS).[1–6] These steels are ultrafine-grained mixtures of ferrite and austenite produced by intercritical annealing, with Mn additions employed to stabilize austenite. Tensile properties are strongly dependent on the fraction and stability of the retained austenite, which in turn is very sensitive to intercritical annealing parameters.[5–9] The fraction and stability of austenite depends on phase transformation behavior and alloy partitioning between the phases during intercritical annealing.[3–5,10] Greater fractions of intercritical austenite (fc) result in reduced stability since less solute partitioning of key austenite stabilizing elements is available. Thus, more of the austenite would transform to martensite during final quenching to room temperature. A straightforward model, assuming ortho-equilibrium alloy partitioning between ferrite and austenite during intercritical annealing, has been proposed to predict the SINGON KANG, Research Associate, EMMANUEL DE MOOR, Assistant Professor, and JOHN G. SPEER, John Henry Moore Distinguished Professor of Metallurgical and Materials Engineering, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected]. Manuscript submitted September 13, 2014. Article published online December 19, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
retained austenite fractions ( fcR ) in medium-Mn steels as a function of intercritical annealing temperature.[11] Applications of the model to a 0.11C-5.7Mn alloy and comparison to experimental data reported by Miller[5,11] for 1-hour isothermal intercritical annealing and to a 0.1C-7.1Mn alloy by Gibbs et al.[6] for 1-week annealing showed reasonable agreement, although the experimental maximum fcR and corresponding annealing temperature (TPeak) were both greater than calculated. Lee and De Cooman[12] modified the martensite transformation part of the model to incorporate composition-dependent coefficients in the Koistinen– Marburger equation[13] and austenite grain size effects on the martensite start temperature (Ms). The results obtained from the modified model in a 6Mn-0.3C steel show improved correlation with experimental data for 1hour annealing.[12] The effects of alloy composition on the predicted fcR in medium-Mn steels have not been systematically investigated and are the focus of the present study for alloy additions of Mn, C, Al, Si, and Cr. The selected base alloy composition is 5Mn-0.2C and the method reported in Reference 11 was applied for the prediction of fcR . Equilibrium fractions of selected phases (austenite, ferrite, and cementite) and their chemical compositions were calculated using the ThermoCalcÒ SSOL 2 and TCFE 7 databases. The amount of martensite formed during quenching from the intercritical annealing temperature was estimated by the Koistinen–Marburger (K–M) equation: fM ¼ 1 e0:011ðMS TÞ
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