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Kermanpur, A.,Behjati, P.,Han, J.,Najafizadeh, A.,Lee, Y.K. Scientific and Technical Press ; Elsevier Science 2015 Materials & Design Vol.82 No.-
Deformation mechanisms of the Ni-free Fe-18Cr-12Mn-0.05C (wt%) austenitic steels containing 0, 0.25 and 0.44N were investigated. Effects of N addition, strain and temperature on the formation of deformation-induced ε- and α'-martensite phases were determined using transmission electron microscopy and electron back-scattered diffraction techniques. Based on the microstructural evolutions, it is proposed that deformation mechanism of these steels includes the sequence of γ→ε-martensite→α'-martensite, in which the formation and stability of the ε-martensite is significantly enhanced by the N content. Increased deformation temperature from 298 to 373K reduces the formation of ε-martensite (more than 80%).
Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Samaei Baghbadorani, H.,Karjalainen, L.P.,Jung, J.G.,Lee, Y.K. Scientific and Technical Press ; Elsevier Science 2014 MATERIALS AND DESIGN Vol.63 No.-
A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C-18Cr-12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173K (900<SUP>o</SUP>C) to achieve the yield strength of minimum 1GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010MPa, tensile strength of 1150MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ~2400MPa at a high true strain of 40% due to slow α'-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.
Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Baghbadorani, H.S.,Jung, J.G.,Lee, Y.K. Elsevier Sequoia 2014 Materials science & engineering. properties, micro Vol.614 No.-
In this work, the influence of precooling on deformation-induced α'-martensitic transformation and mechanical properties of Fe-16Mn-4Cr-0.1C (wt%) austenitic steel is investigated. It is shown that precooling in liquid nitrogen enhances the volume fraction of ε-martensite. During tensile testing, ε-martensite acts as a nucleation site for α'-martensite and promotes the strength and ductility of the steel. The precooled steel exhibits an excellent combination of mechanical properties (tensile strength of 1240MPa, uniform elongation of 35% and total elongation of 42%). Further, it is shown that tensile strength and elongation of the steel strongly depend on the deformation temperature. This is explained by the influence of temperature on stacking fault energy (SFE) and accordingly governing deformation mechanisms of the steel.
Behjati, P.,Kermanpur, A.,Najafizadeh, A.,Samaei Baghbadorani, H.,Jung, J.G.,Lee, Y.K. Elsevier Sequoia 2014 Materials science & engineering. properties, micro Vol.610 No.-
In this work, a specific thermomechanical treatment was used to enhance mechanical properties of a high-manganese austenitic steel. Simple uniaxial tension tests revealed that the treated steel exhibits an outstanding strength (yield strength: 970MPa and tensile strength: 1384MPa) and ductility (fracture strain: 37%) synergy. Microstructural studies showed that several strengthening mechanisms contribute to mechanical properties of the treated steel, including nano grains, nano twinned austenite grains, nano carbides and TRIP. Stacking fault energy and strain-induced martensitic transformation were found to have an important role in appropriate adjustment of the strengthening mechanisms to achieve the optimum properties.
D. Rasouli,A. Kermanpur,E. Ghassemali,A. Najafizadeh 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.4
The martensite reversion treatment was conducted on two grades of Ni-free austenitic stainless steels interstitially alloyed withC and N. The hot rolled sheets of steels were cold rolled up to 80% thickness reduction to acquire strain-induced α′-martensiteand subsequently reversion annealed at temperatures from 700 to 850 °C for 1 to 1000 s to revert the α′-martensite to austenite. Microstructural evolution was investigated using optical microscopy, X-ray diff raction, electron backscatter diff raction,and magnetic measurement techniques. Mechanical properties were measured using tensile tests at room temperature. The resultant microstructures contained both reverted and recrystallized austenite when reverted at 700 and 750 °C withthe annealing time less than 100 s. A nonuniform grain structure was characterized under these conditions consisting ofnano/ultrafi ne grains formed via α′-martensite reversion and coarser grains by recrystallization of the retained austenite. However, a more uniform austenite grain size with average size of 1 μm was obtained at 850 °C for 1000 s. The specimenshaving nonuniform grain structure exhibited excellent combinations of strength and ductility. A variety of mechanical propertieswas achieved depending on the annealing condition. The work hardening behavior aff ected UTS and ductility of thestudied steels. The shift of the work hardening peaks to the higher strains was found suitable for ductility. Addition of C toN-containing Ni-free steels deteriorated mechanical properties. Best combination of strength and elongation was obtainedin the test material with lower C/N ratio.
Lee, C.Y.,Yoo, C.S.,Kermanpur, A.,Lee, Y.K. Elsevier Sequoia 2014 JOURNAL OF ALLOYS AND COMPOUNDS Vol.583 No.-
The effects of the multi-cyclic cold rolling and annealing process on the grain refinement and tensile properties of a metastable austenitic steel were investigated. The multi-cyclic thermo-mechanical process yielded the bimodal ultrafine-grained structure through both the reverse transformation and recrystallization during annealing. The kinetics and temperatures of the reverse transformation were related to the average size of coarse grains. Not only yield and tensile strengths but also ductility of the bimodal-grained structure increased with the number of thermo-mechanical cycles.