Structure and mechanical properties of newly-developed high-strength TRIPLEX type steels

The paper presents the results of mechanical properties research, fractographic investigations of fractures and microstructure of experimental high-strength high-manganese X98MnAlSiNbTi24–11 and X105MnAlSi24–11 TRIPLEX type steels. In order to determine the mechanical properties of the tested steels, microhardness tests and a static tensile test were performed, and yield stress, tensile strength and elongation of the tested steels were determined. The performed microstructure tests of the analysed steels using light microscopy allowed for the identification of austenitic-ferritic structure with the participation of carbides. It was found that the structure of both tested steels, which have undergone hot rolling followed by water cooling, consist of austenite grains with numerous annealing and deformation twins along with ferrite bands. The investigated steels are characterized by the mixed fractures of formed after a static tensile test after forging where there are areas of ductile fracture with small areas of transcrystalline and intergranular brittle fractures. The fractures after a static tensile test and after hot rolling and cooling in water have a dimple morphology characteristic of plastic fractures. The increase in hardness after thermomechanical treatment and after a static tensile test is caused by strain hardening affecting the achieved values of strength, yield point and hardness. The obtained research results allow to assess the impact of both the chemical composition and the applied thermomechanical treatment technology on the properties of newly developed steels