Microstructure and mechanical property evolution in 36MnB5 hot-stamping steel via multi-step tempering–forming: Achieving strength–ductility–residual stress synergy

Abstract

To address the limitations of low ductility and high residual stress in conventional 36MnB5 hot-stamping steel, this study investigates the effects of multi-step tempering strategies on microstructure evolution and mechanical behavior. Three distinct heat treatment routes—quenching-forming (QF), once-tempering-forming (Q1TF), and triple-tempering-forming (Q3TF)—were systematically compared. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) were employed to analyze the retained austenite content and carbide morphology. The QF process delivered the highest ultimate tensile strength of approximately 2372 MPa, but exhibited poor ductility, with elongation limited to 3.2 %, and retained austenite content of less than 0.5 %. The Q1TF process enhanced ductility to 8.0 % and increased retained austenite content to around 2.1 %, though it resulted in reduced strength. Notably, the Q3TF process achieved a desirable combination of properties, including a tensile strength of 2305 MPa, elongation of 8.24 %, and retained austenite content close to 3.0 %, along with a significant reduction in residual stress to 394 MPa. These improvements are attributed to effective stabilization of retained austenite and refined dispersion of carbides through controlled multi-step tempering. The study elucidates the underlying strengthening and toughening mechanisms associated with multi-step tempering and offers a viable pathway for optimizing the balance of strength, ductility, and residual stress in ultrahigh-strength steels. This work provides valuable insight for the design of advanced hot-stamping steels for crash-resistant automotive components demanding both mechanical robustness and excellent formability.