Development of new Si-rich bainitic steels for hot rolling practises

This research develops high-silicon (1.5–2.5 wt.%) carbide-free bainitic (CFB) steels for hot-rolled automotive components, targeting ultrahigh strength (>1400 MPa) and improved mechanical stability over conventional 1000 MPa steels. By tailoring compositions (C: 0.25–0.30 wt.%, Si: 1.52–2.54 wt.%, Mn: 1.26–2.03 wt.%, Cr: 1.22–1.67 wt.%, Mo: 0.22–0.24 wt.%) and coiling temperatures (310°C vs. 350°C), bainitic microstructures with retained austenite were achieved via industrial hot-rolling processes. Elevated silicon suppressed cementite, enabling refined microstructures during bainitic transformation. Hot-rolled strips (3 mm and 12 mm thickness) underwent controlled cooling to prevent ferrite/pearlite formation, followed by coiling simulations. Microstructural analysis revealed bainitic ferrite (60–95 vol%), retained austenite (2–17%), fresh martensite (3–20%), and tempered martensite (0–13%). Lower coiling temperatures (310°C) increased bainite and tempered martensite while reducing fresh martensite. Retained austenite carbon content rose from 0.7–0.8 wt.% (350°C) to 0.8–1.0 wt.% (310°C), enhancing stability. Mechanical testing demonstrated ultrahigh strength (UTS: 1409–1644 MPa) with 9–19.3% elongation. Strength correlated linearly with the composite parameter VCρα/tα. Impact toughness exceeded 27 J at -40°C, with fracture modes transitioning from mixed (-100°C) to fully ductile (+100°C). Kahn tear tests highlighted superior crack resistance at 310°C. The F3 steel (310°C coiling) achieved optimal properties: ~1206 MPa yield strength, ~1563 MPa UTS, and ~19.3% elongation, meeting automotive demands for high strength and energy absorption.