Enhancing mechanical properties of hot-rolled Al-alloyed medium-Mn steel by novel double-step intercritical annealing

The novel heat treatment strategy for advanced medium-Mn steel is presented in the work. Conventional intercritical annealing (IA) was followed by short IA to improve the strength-ductility product of Al-alloyed medium-Mn steel without significant prolonging the duration of heat treatment process. The proper design of the new heat treatment schedules required both experimental and modelling approaches to optimize the microstructure and mechanical properties of investigated steel. The secondary IA step was performed in a temperature range from 750 degrees C to 850 degrees C. The obtained microstructures are composed of ferrite and fine lathtype austenite in different proportions. Specimens annealed at 800 degrees C and 850 degrees C display also some fraction of martensite. No martensite was observed for the specimen held at 750 degrees C. An increase of second IA temperature resulted in a gradual increase of tensile strength due to a higher martensite fraction and a reduced fraction of ferrite in the microstructure. The optimal martensite-austenite ratio resulted in combination of high strength and moderate plasticity for the specimen heat treated at 800 degrees C. The strength-ductility product was about 20.5 GPa%. The C and Mn partitioning from ferrite to austenite and dissolution of cementite are main factors affecting the stability of retained austenite during second IA step and work hardening behavior. The tensile behavior of specimens after two-step IA was affected by serrations resulting from the Portevin-Le Chatelier (PLC) effect.