Effects of ausforming temperature on bainite transformation, microstructure and variant selection in nanobainite steel

Effect of prior martensite on bainite transformation in nanobainite steel

Quantitative analysis of variant selection in ausformed lath martensite

Effect of ausforming on nanobainite steel

A new approach to quantitative analysis of bainitic transformation in a superbainite steel

Bainitic transformation behavior studied by simultaneous neutron diffraction and dilatometric measurement

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 °C leads to ductile fractures with 72% and 95% tensile elongation, respectively, whereas only 25% elongation is achieved for the specimen annealed at 950 °C, exhibiting predominant intergranular facets. 3D atom probe tomography reveals that annealing at 950 °C promotes considerable Mn segregation at the ferrite/austenite phase boundaries with a peak composition of ≈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.

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

Microstructure heterogeneity has been regarded as being detrimental in obtaining reliable mechanical performance of steels. However, in the present study, we demonstrated that a proactive control of microstructure heterogeneity could deliver unprecedented tensile properties that was hardly achieved by using chemically homogeneous initial microstructure. The heterogeneity of Mn distribution generated by utilizing its solubility difference between ferrite, austenite and cementite at intercritical annealing, promoted the retention of austenite in the final microstructure subjected to the room quenching and partitioning process. The enhancement of fraction as well as the stability of austenite contributed to the simultaneous improvement of tensile strength and ductility which have been regarded as mutually exclusive properties. Furthermore, even in steel with lean Mn composition, the room temperature quenching and partitioning process combined with the chemically heterogeneous initial microstructure presented tensile properties comparable to those expected in steels with much higher Mn content, which exhibited the potential of heterogeneity-driven microstructure control for the development of advanced steel products. 

Min-seo Koo

Papers

Effect of ausforming on nanobainite steel

Bainitic transformation behavior studied by simultaneous neutron diffraction and dilatometric measurement

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

Room temperature quenching and partitioning (RT-Q&P) processed steel with chemically heterogeneous initial microstructure