Low-density nano-bainitic Al–C steels

This study investigates the bainitic transformation and mechanical performance of five medium- to high-Al steels with varying C contents, alloying elements (Ti, Ni), and austenitizing/austempering parameters. In steels containing more than 6 wt.% Al, the fastest transformation was determined for the S1 steel with 6.00 wt.% Al at the austempering temperature of 350 °C, with a transformation time of 1600 seconds. In contrast, for the S3 steel with 8.08 wt.% Al, the transformation time increased dramatically, ranging from 14,000 to 43,000 seconds depending on the austempering rate. This significant delay is attributed to the presence of excessively stabilized retained austenite, resulting from C-enrichment around polygonal ferrite. Conversely, S5, with lower C (0.60 wt.%) and moderate Al (3.92 wt.%) levels, exhibited the fastest transformation kinetics (700 – 1000 s) and a uniform bainitic microstructure. Additionally, lenticular martensite near the Ms temperature further accelerated the transformation at lower austempering temperature. Regarding mechanical properties, the ultimate tensile strength (UTS) ranged from 1315 MPa to 1600 MPa. Ti-micro-alloyed S4 achieved the highest average total elongation (10 %) under optimal heat treatment. Yield strength (R p0.2 ) trends generally matched the UTS, except for S3 at low austempering temperature. Microstructural precipitate analysis confirmed the primary formation of (FeM) x C y and TiMoC. MatCalc simulations supported these findings, indicating primary precipitation above austempering temperatures at the austenitizing and cooling stages. Overall, the findings highlight the pivotal role of alloy composition and microstructural features in governing bainitic transformation and the resulting strength–ductility response.