High Temperature Deformation-Induced Transformation in Nb-Bearing Medium Mn Steel
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ium manganese steel is currently being evaluated as a possible third generation ultra-high strength alloy concept for future lightweight passenger vehicles.[1] Lee and De Cooman[2–4] have shown that cold rolled medium manganese steel achieves impressive strength-ductility properties, e.g., the product of ultimate tensile strength (UTS) and total elongation (TE) beyond 50,000 MPa pct, after intercritical annealing. As medium manganese steels are characterized by a pronounced hardenability, the microstructure is very often fully martensitic after hot rolling, making an intermediate heat treatment necessary to obtain a softer microstructure prior to cold rolling.[5,6] As an alternative strategy, the addition of aluminum in medium manganese steel is considered to soften the hot rolled steel without an intermediate heat treatment.[7,8] The austenite is partly replaced by stabilized d-ferrite at the hot rolling temperatures. The d-ferrite experiences no phase
WOOJUN KIM, SINGON KANG, and BRUNO C. DE COOMAN are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, South Korea. Contact email: [email protected] BIAN JIAN is with the CBMM Asia PTE Ltd, 10 Collyer Quay, Singapore, 049315, Singapore. SEA WOONG LEE is with the Steel Products Research Group, Technical Research Laboratories, POSCO, Gwangyang 57807, South Korea. Manuscript submitted July 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation during and after the hot rolling, thereby reducing the final martensite fraction. Considering the fact that deformation may promote the dynamic phase transformation of austenite into ferrite,[9] hot rolling at lower temperatures can contribute to the softening of the hot rolled specimen. The deformation-induced transformation of deformed austenite to ferrite at temperatures above the Ae3 transformation temperature is referred to as dynamic transformation (DT). It has been reported to occur during the hot deformation of different alloys, including low C steels[10,11] and Fe-6Ni-xC alloys (in wt pct).[12,13] Ghosh et al.[14,15] reported that the DT to ferrite typically takes place at temperatures of about 100 °C above Ae3. Thus, the DT ferrite can undergo transformation to austenite during following holding at the temperature for the DT.[16–18] According to Ghosh et al.[14] in the alloys with a carbon content less than about 0.4 wt pct, DT occurs in a displacive manner and is not accompanied with the partitioning of alloying elements during the transformation. Aranas and Jonas[19] analyzed the effect of manganese additions in the range of 0.01 to 1.4 wt pct on the dynamic transformation of Fe-Mn-Si steel containing less than 100 ppm carbon. While the addition of manganese increases the driving force for the dynamic transformation to ferrite, its effect on the DT is somewhat ambiguous, because the increased free energy difference between ferrite and austenite enhances the austenite stability. Research data on the DT behavior in alloys contai
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