A novel technique to produce trimodal microstructure in low-carbon steel

In this research, a novel technique (intercritical annealing, controlled cooling, cold rolling, and final annealing) to produce trimodal microstructure in low-carbon steel is proposed. After intercritical annealing, a cooling rate was used (30 °C/s) which led to the simultaneous formation of ferrite, martensite, and pearlite phases. After cold rolling (with the strains of 25 %, 50 %, and 75 %) and final annealing at 550 °C, recrystallization had not yet occurred in the microstructure of the 25 %+550 and 50 %+550 samples, while the microstructure of the 75 %+550 sample had completely undergone static recrystallization. A trimodal microstructure consisting of coarse grains (larger than 5 μm), fine grains (between 1 and 5 μm), and ultrafine grains (less than 1 μm) was formed in the 75 %+550 sample. Among the annealed sheets, the 75 %+550 sheet exhibited the highest hardness (217.5 HV), yield strength (686.1 MPa), and tensile strength (709.5 MPa), with an acceptable total elongation (11.3 %) due to the formation of trimodal microstructure and fine spherical θ-Fe₃C particles. By increasing the strain (in both deformed and annealed samples), the central area became smaller, indicating a shift toward shear ductile fracture. The final annealing at 550 °C slightly increased the central area owing to the occurrence of restoration mechanisms.