Abstract
The primary objective of this study was to determine how intercritical annealing affects the martensite start temperature and subsequent low-temperature bainitic transformation in a 0.17C-3.1Mn-1.6Al-0.2Si-0.2Mo-0.04Nb medium-manganese multiphase steel. A two-step heat treatment was designed, consisting of intercritical annealing in the range of 780-870°C, followed by isothermal holding at 200-300°C. The transformation behaviour was analysed using dilatometric measurements, thermodynamic calculations and scanning electron microscopy. Increasing the intercritical annealing temperature from 780°C to 870°C increased the martensite start temperature from approximately 170°C to 305°C. Lower annealing temperatures promoted stronger carbon and manganese enrichment of austenite, but reduced the amount of austenite available for bainitic transformation. The dilatometric response during isothermal holding increased with higher annealing and holding temperatures. Martensitic transformation during final cooling was detected after intercritical annealing at 780°C followed by isothermal holding at 200°C, and after intercritical annealing at 800°C followed by isothermal holding at 220°C. The most favourable response in terms of forming refined bainitic products was obtained after intercritical annealing at 860 or 870°C followed by isothermal holding at 300°C, where measurable bainitic transformation was combined with no clear martensitic signal during final cooling. The proposed route is therefore promising for further optimisation, although high-resolution characterisation is still required to confirm nanobainitic morphology and quantify phase fractions.