Engineering Carbide-Free Bainitic Steels with High Silicon for Superior Strength and Toughness

This research presents the development and comprehensive evaluation of high-silicon (1.5–2.5wt.%) carbide-free bainitic (CFB) steels engineered for industrial hot rolling, targeting coiling temperatures of 310℃ and 350℃. Designed to meet the automotive industry’s demand for materials with both ultrahigh strength and enhanced toughness, these novel steels utilize elevated silicon content to suppress cementite formation and optimize bainitic microstructures with retained austenite. Four distinct steel compositions, produced via vacuum induction melting, were hot rolled to final thicknesses of 3mm and 12 mm under industrially relevant cooling regimes. Microstructure characterization by SEM and XRD revealed bainitic ferrite fractions of 60–95%, with variable amounts of retained austenite (2–17%), fresh martensite (3–20%), and tempered martensite (0–13%), depending on coiling temperature. Lower coiling at 310℃ promoted finer microstructures with higher hardness and carbon-enriched austenite. Mechanical testing demonstrated ultimate tensile strengths of 1409–1644 MPa and total elongations up to 19.3%. The steels exhibited excellent low-temperature impact toughness, exceeding 27 J at –40℃, and superior notch resistance and crack initiation delay in Kahn tear tests, especially for samples coiled at 310℃. The research successfully identifies processing parameters and alloy designs for CFB steels that achieve an exceptional synergy of strength, ductility, and toughness—attributes highly desirable for next-generation automotive structural components.