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Improvement of Impact Toughness of 5Mn-1Al-0.5Ti Steel by Intercritical Annealing
이일철,하유미,이학철,A. Zargaran,Nack J. Kim 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.2
The present study is aimed at improving the impact toughness of 5Mn-1Al-0.5Ti steel by incorporating ferrite-martensite dual phase microstructure by intercritical annealing. Although (8-12)Mn martensitic steels usuallyshow very low impact toughness due to the occurrence of intergranular fracture, the martensitic structure ofthe present 5Mn-1Al-0.5Ti steel fails by transgranular cleavage fracture due to higher grain boundary strengththan matrix strength incurred by reduced Mn content and segregation of Ti along grain boundaries. Nevertheless, itstill shows very poor impact toughness at room temperature due to its coarse grain size. The application of intercriticalannealing, i.e., formation of dual phase microstructure, is shown to significantly decrease ductile-to-brittletransition temperature (DBTT), with only a small degradation of tensile properties; however, microstructuralexaminations show that most of ferrite/martensite interfaces have a character of low angle boundaries andtherefore such decrease in DBTT is not necessarily due to the formation of ferrite-martensite dual phase structure,but rather to the refinement of grain size by low temperature annealing.