Thermodynamic-guided design of cost-effective ultrahigh-strength carbide-free bainitic steel

Carbide-free bainitic (CFB) steels offer an attractive combination of strength, ductility, and toughness but generally rely on complex alloy chemistries and prolonged isothermal treatments to achieve bainite formation and retained-austenite stabilisation. Here, a new air-cooled CFB steel with simplified chemistry was developed using a thermodynamics-guided alloy design framework. By analysing the effects of common alloying elements on the bainite start temperature, T 0 ′ locus, and bainite hardenability, the carbon partitioning window is introduced as a thermodynamic descriptor to assess the compatibility of bainite formation under continuous cooling. Using this framework, it is shown that combined additions of cost-effective Mn, Al, and Si are sufficient to promote carbide-free bainite without relying on conventional heavy alloying with Ni, Cr, or Mo. Based on this design strategy, a Fe-0.4C-3.5Mn-2Si-0.5Al steel was selected and experimentally validated. The alloy formed a fully CFB microstructure directly after forging and air cooling, eliminating the need for prolonged isothermal treatment. The as-forged steel exhibits a yield strength of ∼1650 MPa, an ultimate tensile strength exceeding 2300 MPa, an elongation greater than 8%, and a Charpy impact toughness of 14 J. These properties are comparable to those of heavily alloyed CFB and martensitic steels while requiring substantially simpler chemistry and processing. The proposed design framework provides a practical pathway for developing cost-effective, sustainable ultrahigh-strength bainitic steels.