Effects of Mn Segregations on Intergranular Fracture in a Medium-Mn Low-Density Steel

초록

Al-bearing medium-Mn low-density steels possess great potential in the automotive industry because of their excellent mechanical properties based on transformation-induced plasticity and low specific weight. Reducing the austenite stability against deformation-induced martensitic transformation enables a high strain-hardening capacity to be obtained; however, undesirably low stability often results in considerably reduced tensile ductility and brittle fracture. Herein, the brittle fracture that occurs with increasing annealing temperature for a Fe-0.3C-9Mn-5Al (wt%) steel is investigated in relation to Mn segregation at the phase boundaries between ferrite and austenite. The results demonstrate that annealing at 850 and 900 degrees C leads to ductile fractures with 72% and 95% tensile elongation, respectively, whereas only 25% elongation is achieved for the specimen annealed at 950 degrees C, exhibiting predominant intergranular facets. 3D atom probe tomography reveals that annealing at 950 degrees C promotes considerable Mn segregation at the ferrite/austenite phase boundaries with a peak composition of approximate to 19 at%, which is sufficient to reduce the boundary cohesion for intergranular fracture. Thermodynamic moving boundary simulation reveals that intercritical annealing is not a prerequisite for segregations; however, low-temperature and prolonged holding should be accompanied, such as the coiling procedures.

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