Enhancing strength without sacrificing ductility in medium-carbon multiphase bainitic steel through increased intercritical annealing temperatures

In bainitic steels, enhancing strength often comes at the expense of ductility. In this paper, a medium-carbon multiphase bainitic steel was produced through intercritical annealing (IA) at various temperatures, followed by isothermal bainite transformation (IBT) at 300 °C, and its microstructural evolution and mechanical properties were systematically investigated. The final microstructure primarily consists of intercritical ferrite, bainite, retained austenite (RA) and fresh martensite. During IA, acicular and globular morphologies of reverted austenite were observed, with acicular austenite exhibiting a near Kurdjumov–Sachs orientation relationship with intercritical ferrite. Higher IA temperatures shorten the incubation and completion times of bainite transformation through two synergistic mechanisms. First, the increased volume fraction of acicular austenite promotes bainite nucleation by lowering the energy barrier. Second, elevated IA temperatures enhance C and Mn diffusion, accelerating the growth of reverted austenite, increasing its size, and reducing the C and Mn concentrations at austenite/ferrite interfaces. These effects alleviate the C/Mn-induced retardation of bainite transformation and accelerate the bainite formation. Moreover, higher IA temperatures lead to a higher bainite fraction and a reduced intercritical ferrite content, both of which contribute to improvements in yield and tensile strength. Higher IA temperatures also increase the RA volume fraction after IBT, enhancing the transformation-induced plasticity (TRIP) effect by promoting austenite-to-martensite transformation. The increased bainite content improves the stress shielding effect on adjacent RA, promoting the mechanical stability of RA. These combined effects are believed to be mainly responsible for the enhanced tensile strength and ductility of the steel.