بررسي رفتار تغيير شكل فولاد چند فازي با آستنيت باقي مانده

Deformation Behaviour of Multi-Phase Steel with Retained Austenite

The main goal of this research was to evaluate the compressive deformation behaviour of multi-phase steel with retained austenite. Following a multi-stage heat treatment cycle on a low-alloy steel with 0.3 wt.%C, a multi-phase steel with considerable amount ofretained austenite was produced. The determination ofretained austenite in steel microstructures was performed by two different methods; image analysis of optical microscopic images and X-ray diffraction. The amount of retained austenite was measured to be around 18% in both techniques. Compressive deformation behaviour of the investigated steel was examined at different temperatures. A very high strength ratio (the ratio offlow stress at high strain value to yield strength) was obtained for the steel at room temperature exhibiting a characteristic combination of low yield strength and high work hardening rate. Achievement of high strength ratio was attributable to the presence of austenite phase in the steel microstructure and its transformation to hard martensite phase during straining (Transformation-Induced Plasticity Effect). The stress-strain response of the investigated steel at higher deformation temperatures was unusual. Flow stress and work hardening rate increased with an increase in temperature and reached their maximum values at the highest deformation temperature. Since retained austenite becomes more stable at higher deformation temperatures, formation of a transformation product other than martensite phase from austenite during straining was considered to be responsible for this unusual deformation behaviour. This transformation product was assumed to have high strength in order to give rise to a considerable microstructural strengthening effect.