Effect of coupled intercritical isothermal holding and rapid heating on bainitic transformation and mechanical properties in medium-carbon nanobainitic steel

A coupled intercritical-isothermal–rapid-heating (II-RH) route is proposed to exploit, rather than eliminate, chemical heterogeneity in a bainitic medium-Mn steel (Fe–0.29C-1.97Si–1.0Ni–1.56Mn–1.45Cr). Intercritical holding at 770 °C establishes C/Mn/Cr partitioning, followed by rapid heating to 910 °C (100 °C/s) and isothermal transformation at 340 °C. Compared with a conventional direct isothermal treatment (M-DIT), II-RH preserves Mn/Cr-enriched and-depleted austenite domains that govern transformation kinetics and morphology. Dilatometry reveals a higher initial bainite transformation rate in II-RH despite similar overall expansion. DICTRA simulations and experiments indicate that Mn/Cr-depleted regions show elevated local Ms (∼441 °C) and undergo interfacial martensite formation during isothermal quenching; the resulting defects reduce the nucleation barrier for bainitic ferrite, effectively removing the incubation period and accelerating growth. Microstructurally, II-RH yields coarser BF laths (0.29 μm vs. 0.24 μm) and a mixed RA morphology (film/short-rod/blocky) with larger average thickness (122 nm vs. 79 nm), while XRD shows comparable RA carbon contents to M-DIT. Mechanically, II-RH achieves a superior strength–ductility synergy with PSE = 38.5 GPa·%, UTS = 1189 MPa, and EL = 32.4 %, whereas M-DIT attains higher strength (UTS = 1592 MPa; YS = 1173 MPa) but lower ductility (EL = 17.4 %; PSE = 27.7 GPa·%). The results demonstrate that leveraging kinetic mismatch between rapid austenitization and sluggish Mn diffusion stabilizes retained austenite and tailors bainite kinetics, providing a practical pathway to design next-generation high-performance bainitic steels.