Cost-effective S690QL steel sheet production: TMCP-based accelerated cooling as an alternative to furnace quenching

This study explores a cost-efficient alternative to conventional furnace quenching for producing 20 mm thick S690QL-grade steel sheets (EN 10025-6) via a combination of Thermo-Mechanically Controlled Processing (TMCP) and Accelerated Cooling (ACC). Two ACC strategies are compared: single-stage ACC, yielding a uniform lath martensitic microstructure with high strength but limited ductility, and two-stage ACC, producing a predominantly bainitic structure with an enhanced strength-ductility balance. The influence of tempering temperature (600–720 °C) on microstructure and mechanical properties under each ACC regime is examined through comprehensive mechanical testing and microstructural characterization (optical and transmission electron microscopy). Property variations are associated with tempering-induced microstructural transformations, including recrystallization, carbide precipitation (cementite, (V,Mo,Cr)C carbide), and their morphological evolution (coalescence/spheroidization). Notably, at higher tempering temperatures (>660 °C), vanadium-based carbides remain nanoscale, whereas cementite grows to 120–200 nm. A resource-saving processing route integrating TMCP, single-stage ACC, and subsequent furnace tempering at 680 °C for 60 min is successfully trailed under industrial conditions, to obtain 20 mm thick steel sheets fully meeting S690QL specifications: yield strength of 740 MPa, tensile strength of 830 MPa, elongation at fracture of 17%, and impact strength (impact energy) of a Charpy V-notch test piece at –40 °C of 85 J. This innovative approach allows eliminating energy-intensive furnace quenching, achieving a substantial reduction of 300 m 3 natural gas consumption per tonne of steel, underscoring a pathway toward sustainable production of high-performance steels.