Effects of heat treatment on evolution of precipitates and mechanical properties of Fe-Mn-Al-C lightweight steel
The evolution of precipitates and its effects on mechanical properties in a Fe-30Mn-9Al-1.2C austenitic lightweight steel were investigated by tuning the heat treatment process.The results show that the lightweight steel which is subject to solid solution and aging treatments after cold rolling exhibits a fully recrystallized austenite structure,characterized by a significant presence of uniformly dispersed fine κ-carbides(grain size of 8 nm)within the grains.The sample demonstrates a superior combination of strength and ductility,with a yield strength of 839 MPa and a total elongation of 57％.However,when the lightweight steel is directly annealed at the aging temperature after cold rolling,partial recrystallization occurs.The yield strength increases to 1 380 MPa whereas the total elongation drops to merely 5.3％,displaying pronounced brittle fracture characteristics.This is mainly attributed to the formation of triplex microstructures including κ-carbides(grain size of 0.4 μm),α-ferrite(grain size of 0.5 μm),and γ0-austenite(grain size of 0.7 μm)in the recrystallized zone through the eutectic reaction.During the tensile deformation process,the strain incompatibility caused by the significant difference in hardness between the various phases leads to the stress concentration at the hard κ-carbide interface,which causes the κ-carbide to fracture or the micro-crack propagating along the interface,accelerating the failure process.
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