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Obtaining a desired quantity of retained austenite (RA) in press-hardened steels (PHSs) with a martensitic matrix is of great interest for the performance of safety critical components in vehicle bodies, as it improves the ductility/toughness without compromising strength. In this study, RA in a 1.7 GPa−PHS was stabilized via local chemical heterogeneity associated with Cr-enriched carbides. The Cr-rich carbide was introduced as a temporary reservoir of Cr and C into the pre-hot stamping microstructure by an annealing treatment. During the subsequent austenitization in hot stamping, the slow decomposition kinetics of Cr-rich carbide contributed to localized enrichment of Cr and C, which stabilized the austenite and resulted in a significant amount of RA with a core-shell structure after die quenching. Finally, sufficient RA ensured that the 1.7 GPa−PHS had a total elongation of ∼9.3%, which is about 43% higher than that of the 1.5 GPa−PHS grade 22MnB5 (∼6.5%). [Display omitted] [ABSTRACT FROM AUTHOR]

A novel press-hardening steel (PHS) with an ultra-high tensile strength of 1680 MPa and an excellent total elongation of 9.2% has been developed and produced at an industrial scale. In addition to high strength, the new PHS showed significantly improved ductility as compared to the baseline PHS grade 22MnB5 (~7.0%) mainly because of its nano-sized retained austenite (RA), which was formed as a result of dynamic carbon partitioning after the martensitic transformation during the hot forming process. Dynamic carbon partitioning from the martensite to austenite during cooling and RA stabilization was achieved by adding optimum amounts of Si and Cr to the novel PHS without alternating the industrial hot forming process. To investigate the effect of the RA on the mechanical performance of the new PHS, microstructure without RA was prepared for comparison by direct water quenching after the austenitization to suppress the carbon migration. Detailed characterization revealed that the RA was beneficial for improving both the uniform and post-uniform elongation of the novel PHS. The effects of Si and Cr on the formation of the RA were investigated with the aid of thermo-kinetic calculations. The excellent combination of strength and ductility of the novel PHS will enable a significant weight reduction when it replaces the current PHS grade 22MnB5 for a variety of vehicle body structure components. [ABSTRACT FROM AUTHOR]

This study aimed to overcome the existing trade-off in strength–ductility in carbide free bainitic steel and improve ductility without compromising strength. Herein, we proposed a novel and simple method to obtain Mn-rich cementite via a pre-annealing (PA) treatment. PA promoted the partitioning of Mn into cementite particles. The Mn-rich cementite increased the Mn content in the retained austenite (RA) and enhanced the RA stability. Moreover, the initial number of Mn-rich cementite microstructures is positively correlated with RA. The effect of PA on the bainitic ferrite (BF) content was evaluated via comparison with the non-PA sample, revealing that the PA treatment promoted BF nucleation and increased the BF content. The PA-treated samples exhibited the optimal transformation induced plasticity (TRIP) effect and mechanical properties; in particular, the PA-3 sample (with 3 h treatment period) improved ductility and formability, with the total elongation increased from 11.9% in the non-PA sample to 18.0% in the PA-3 sample, corresponding to an increase of ∼51.2%. Additionally, the Erichsen cupping value increased from 7.52 mm in the non-PA sample to 9.78 mm in the PA-3 sample, corresponding to an increase of 30.1%. The overall applicability of PA for the improvement of formability of the carbide free bainitic steel is evident in all PA-treated samples. • The PA treatment is a novel and simple way to improve the ductility and formability of the material. • The PA promoted the partitioning of Mn into cementite particles. • The Mn-rich cementite results in Mn enrichment in the retained austenite (RA) and enhanced the RA stability. • The PA treatment reduced fresh martensite content and improved bainitic ferrite content. • Excellent strength and ductility in low-carbon bainitic steel was obtained via short-term PA treatment. [ABSTRACT FROM AUTHOR]