Effect of silicon on phase transformation kinetics and austenite stabilization under twin-roll-strip-casting-relevant cooling and simulated coiling conditions

Twin-roll strip-casting (TRSC) provides a promising low-carbon route for manufacturing thin-gauge ultra-high-strength steels. However, the narrow laminar cooling window and coiling-temperature fluctuations in TRSC lines challenge the microstructural stability of martensitic steels. Inspired by the critical carbide-suppressing role of Si in high-strength quenching and partitioning steels, this work systematically compares the transformation behavior of low-Si and high-Si steels with Si contents of 0.4 and 2.0 wt.% and underlying mechanisms during continuous cooling and isothermal holding (simulated coiling) under cooling conditions relevant to TRSC processing. The results show that, during continuous-cooling transformation, increasing Si raises the critical transformation temperatures, stabilizes ferrite, and slightly widens the intercritical temperature interval; meanwhile, increasing Si significantly broadens the ferrite transformation region, narrows the martensitic transformation region, refines the microstructure, and markedly increases the matrix hardness. During isothermal transformation, more than 10% retained austenite is preserved to room temperature in the 2.0Si steel. X-ray diffraction (XRD) results show that, after holding at 300 °C for 100 s, the retained austenite fraction in the 2.0Si steel reaches 15.08 vol.%; even after prolonged holding at 300 °C for 10,000 s, 11.30 vol.% retained austenite is still retained, with an average carbon content of 1.15 wt.%. Transmission electron microscopy/selected-area electron diffraction (TEM/SAED) characterization further reveals that ε-carbides precipitate in both steels, and the average carbide size is markedly refined in the 2.0Si steel compared with the 0.4Si steel: the average length and width of ε-carbides decrease from 180.11 nm and 25.85 nm in the 0.4Si steel to 106.66 nm and 9.05 nm in the 2.0Si steel, respectively. These results indicate that Si significantly suppresses carbide growth and coarsening, reduces carbon consumption by carbide precipitation, and allows more carbon to enrich the untransformed austenite, thereby enhancing retained-austenite stability and retarding bainitic decomposition during simulated coiling. Therefore, the present low-/high-Si comparison indicates that the high-Si condition is effective in regulating transformation pathways, carbide evolution, and austenite stability in martensitic steels under TRSC-relevant cooling–coiling conditions, providing a phase-transformation basis for composition design and microstructure control of thin-gauge high-strength martensitic steels.