Enhancing the yield strength of intercritically annealed Q&P steel via bainite-based quenching and partitioning treatment

Abstract

Intercritically annealed Q&P steels exhibit good strength-ductility synergies. Unlike fully austenized Q&P steels, intercritically annealed Q&P steels exhibit good ductility but contain a significant amount of ferrite, resulting in a lower yield strength of the material. To address this challenge, we propose a novel short-time isothermal method for bainite-based quenching and partitioning (B-Q&P) treatments to enhance the yield strength. The experimental results indicate that a short-term isothermal bainitic transformation generates a small quantity of preformed bainitic ferrite (PFB), accelerating primary martensite (PM) transformation and enhancing the PM content during the initial quenching stage. Moreover, the B-Q&P treatment significantly refines the retained austenite (RA) and improves its mechanical stability, thus resulting in a more sustained transformation induced plasticity (TRIP) effect. Furthermore, the results reveal that the yield strength increases from 632 MPa in the conventional Q&P sample to 792 MPa in the B270-Q&P sample, representing a ∼25.3 % improvement, the total elongation increased from 22.4 % in the conventional Q&P sample to 24.1 % in the B270-Q&P sample. Additionally, the impact of the bainite-based quenching temperature on the content and transformation kinetics of the primary martensite is examined.

Keywords

• Intercritically annealed Q&P steels
• B-Q&P treatment
• Preformed bainitic ferrite
• Primary martensite
• Yield strength
• Retained austenite