چكيده به لاتين
During the past decades, the automobile industry has devoted tremendous effort to investigate advanced high strength steels (AHSS) with multiphase microstructure for the purpose of improving fuel efficiency and safety. Transformation-induced-plasticity (TRIP) steel has received much attention in the automotive body construction due to its outstanding combination of strength and ductility.
The purpose of this study was to investigate the effect of isothermal bainitic transformation temperature and time on microstructure and mechanical properties of a -TRIP steel.In this study, the steel with chemical composition of 0.25C-0.33Si-1.05Mn-4.56Al was heat treated after being cast and rolled in order to make steel TRIP. Primarily, each specimen was austenitized at the intercritical annealing temperature of 820°C for 10 min and then was heat-treated in salt bath with different temperature 350- 400- 450 °C for different duration of 2, 6 and 10 min, respectively. The microstructure of each sample has been investigated by optical microscope (OM) and scanning electron microscope (SEM). The microstructure of steel consists -ferrite, bainite, residual austenite, and small amounts of martensite after heat treatment. The volume fraction of residual austenite and it’s containing carbon were measured by X-Ray diffraction (XRD) analysis. The results showed that by increasing in the isthothermal bainitic transformation temperature, the volume fraction of residual austenite increases in steel and the amount of carbon in the residual austenite decreases. Also, by increasing the holding time of steel at the isthothermal bainitic transformation temperature, the volume fraction of the residual austenite initially increases and then decreases which cause to increasing the amount of carbon contained in the residual austenite. The mechanical properties of steel were investigated by tensile test. The best combination of strength and ductility obtained in the sample which was heat treated at 450 °C for 6 min. This sample has the tensile strength (UTS) of 1046 Mpa and elongation (EL) of 54%.
Keywords: TRIP Steel, Heat Treatment, Mechanical Properties, Microstructure, Residual Austenite.