Influence of Nb-Microalloying on the Formation of Nano/Ultrafine-Grained Microstructure and Mechanical Properties During
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THERE is a great and continued interest in developing new generation of advanced high strength steels including stainless steels, with a high ultimate tensile strength above 1 GPa and good tensile elongation of 15 to 30 pct for lightweight applications. In this context, grain refinement down to the submicron size leading to the nano-grained (NG) or ultrafine-grained (UFG) microstructures has been suggested as a viable approach to improve strength without significant loss in ductility.[1,2] It is well known that in metastable austenitic steels, the strain-induced a¢-martensite reverse transformation (SIMRT) process enables a marked grain refinement.[3– 8] This process has been successfully applied to refine microstructure of Fe-Cr-Ni commercial austenitic stainless steels (3XX series) such as 301,[3,5,9] 301LN,[10] 304,[11] 304L,[12] and 316L[13] grades. In recent years, the uncertainty in Ni price and its allergenic effect has HOJJAT SAMAEI BAGHBADORANI, PEIMAN BEHJATI, MOHAMMAD MOALLEMI, and AHAD REZAEE, Researchers, and AHMAD KERMANPUR, Associate Professor, are with the Department of Materials Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran. Contact e-mail: h.samaeibaghbadorani@ ma.iut.ac.ir ABBAS NAJAFIZADEH, Professor, formerly with the Department of Materials Engineering, Isfahan University of Technology, is now with the Fould Institute of Technology, 84916-63763 Fouldshare, Iran. Manuscript submitted June 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
motivated studies on Fe-Cr-Mn-low Ni austenitic stainless steels (2XX series) such as 201,[14,15] 201L,[16] and 204Cu[14–17] grades. It is well established that microalloying elements (including Ti, Nb, and V) play an important role in controlling grain growth and accordingly mechanical properties of heat-treated steels.[18–20] This suggests that the microalloying elements can be used to maintain the nano/ultrafine-grained microstructure achieved in the initial times of reversion annealing and hence to prevent abrupt grain growth, which is usually observed with continued annealing in SIMRT processed steels.[15] Despite extensive studies on different aspects of SIMRT process, the influence of microalloying on evolution of microstructure and mechanical properties during martensite reversion has not received enough attention. In a recent study, the SIMRT process was used for the formation of NG/UFG structure in a 201L austenitic stainless steel containing 0.12 wt pct Ti-microalloying element.[21] The results showed a promoting effect of Ti addition on the formation of strain-induced a¢-martensite (SIM). In addition, it was found that precipitation of nano-sized Ti-rich carbides during the reversion annealing could retard the reversion process and suppress grain growth in further annealing times. Among all microalloying elements, Nb is often considered to be the most powerful element in retarding the growth of austenite grains.[22] Lee et al.[23] found that the nanosized Nb-rich carbides increased the strength without
reducing the ductil
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