Effect of Cr content on hot ductility of low-alloy high-strength steel

This study investigates the hot ductility of SAPH440 low-alloy high-strength steels with varying chromium (Cr) contents (0Cr, 0.6Cr and 1.5Cr) using the Gleeble-3500 thermal simulation testing machine and the DIL805A thermal dilatometer. Post-fracture microstructures were analysed using scanning electron microscopy, and the distribution of chromium was examined via electron probe microanalysis. The results show that the lowest reduction of area (RA) of 51.93% for 0Cr steel occurs at 780°C, within the third brittle zone. The addition of chromium at 0.6% and 1.5% effectively lowers the temperature and narrows the range of the third brittle zone, with the RA exceeding 60% compared to the 0Cr steel. While chromium acts as a thermodynamic ferrite stabiliser, it effectively lowers the phase transformation start temperature ( Ar 3 ) by increasing austenite hardenability and suppressing carbon diffusion. This inhibits the formation of detrimental pro-eutectoid ferrite films at grain boundaries, shifting the microstructure towards a uniform lath bainitic matrix accompanied by deformation bands, which provides enhanced deformation coordination. Moreover, chromium strengthens the ferrite phase and reduces the strength mismatch between the two phases, thereby optimising deformation coordination and significantly improving hot ductility. These results highlight chromium's critical role in enhancing the high-temperature performance of low-carbon mild steel, offering theoretical insights for optimising scrap steel recycling.