| As a new type of high strength steel,not only does Fe-Mn-Al-C steel has excellent mechanical properties,but also combines the characteristics of low density and corrosion resistance,which shows great potential application in automotive structural lightweight,crash safety and complex working conditions.Considering the significant influence of heat treatment on the microstructures and properties of Fe-Mn-Al-C steels,this work investigated the hardening response,microstructures and mechanical properties of lowdensity austenitic Fe-30Mn-10Al-2C steel which had a density of 6.44g/cm3 during aging and solution treatment.To improve the strength of present steel,aging treatment was employed.Based on the aging temperature,the difference in microstructures and mechanical properties was observed.Owing to the precipitation of κ-carbide during low-temperature aging of 500-550℃,though severe plastic loss was accompanied by the aging treatment,the strength and ductility was improved remarkably.However,there was no distinct precipitation at grain boundaries,low-temperature aging was thought as potential processing to improve mechanical properties of present steel.For the medium-temperature aging at 600-650℃,continuous novel hardening response could be observed,which originated from the transformation of β-Mn.Although the improvement of strength was brought,β-Mn caused critical loss in ductility,even the embrittlement due to irregular morphologies and positional distribution.When aging was conducted at high temperature ranging from 700℃ to 800℃,the balance between aging hardening and high-temperature softening was built leading to approximated constant hardness.Unfortunately,the α particles distributed along grain boundaries were detrimental for the ductility of the steel.During aging of 500-700℃,β-Mn formed along grain boundaries.However,the formation kinetics of β-Mn were different depending on the aging temperature.The medium-temperature was better for the transformation of β-Mn.Due to the unfavorable effects on properties,the β-Mn transformation behavior was discussed based on the diffusion and redistribution of elements.In addition,the diagram of β-Mn transformation was built,and the microstructures were divided based on the diagram of β-Mn transformation to establish the foundation for the choice of aging parameters and hotworking processing.To improve the plasticity of the present steel,the effect of solution temperature was investigated.The result suggested that solution temperature affected significantly the microstructure and properties of experimental steel.With the increase of solution temperature,the plasticity was greatly improved,while the strength decreased slightly.The products of strength and plasticity of samples treated at 1100℃ and 1200℃ for 1h approached 46.0 GPa·% and 42.6 GPa·% respectively.But,when solution was carried out at the temperature above 1100℃,the transformation of high-temperature ferrite happened,and high-temperature ferrite was retained in microstructure during aging.After aging treatment,the strengths of samples treated at different solution temperature were improved.However,the ductility of all samples especially the sample treated at 1200℃ decreased,which attributed to the retained high-temperature ferrite.As a result,the sample treated at 1200℃ showed a lower product of strength and plasticity of 31.2 GPa·%,while the sample treated at 1100℃ maintained a high product of strength and plasticity with 44.9 GPa·%,which means better mechanical properties.Through the observation and analysis,samples aged at 500℃ for 4h after solution treated at 1100℃ for 1 h had a great combination of properties.Thus,the treatment of 1100℃/1h + 500℃/4h was optimized heat treatment parameters. |
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