Search

Al-Si-coated boron-alloyed steels are the most widely used press-hardened steels (PHSs), which offers good oxidation resistance during hot forming due to the presence of the near eutectic Al-Si coating. In this study, a recently developed novel un-coated oxidation resistant PHS, called coating-free PHS (CF-PHS), is introduced as an alternative to the commercial Al-Si coated PHSs. With tailored additions of Cr, Mn, and Si, the new steel demonstrates superior oxidation resistance with a sub-micron oxide layer after the conventional hot stamping process. Hence, it does not require shot blasting before the subsequent welding and E-coating process. Two CF-PHS grades have been developed with ultimate tensile strengths of approximately 1.2 and 1.7 GPa, respectively. Both grades have a total elongation of 8–9%, exceeding the corresponding Al-Si-coated PHS grades (1.0 GPa/6–7%, 1.5 GPa/6–7%). Furthermore, the bendability of CF-PHS was similar to the corresponding Al-Si PHS grades. On the other hand, performance evaluations relevant to automotive applications, such as weldability, the E-coat adhesion, and tailor-welded hot stamp door ring, were also conducted on the CF-PHS steel to satisfy the requirements of manufacturing. [ABSTRACT FROM AUTHOR]

We explored the feasibility and benefits of rapidly heating (at over 100°C/s) a novel coating-free oxidation-resistant press-hardening steel (PHS). Experiments indicated that a total heating time of 120 s was sufficient to achieve complete austenitization at 930°C with a heating rate of 100°C/s for a 2-mm-thick coating-free PHS blank. Tensile testing results showed that ultimate tensile strength of 1722 MPa and uniform elongation of 5.1% were obtained after rapid heating, soaking, water quenching, and baking, representing a significant improvement over values for the baseline PHS 22MnB5 (1583 MPa and 4.1%) subjected to the same treatment. In addition, the thickness of the oxide layer on the coating-free PHS after rapid heating and soaking (120 s) in ambient atmosphere was less than 5 μm, being much thinner than that on the 22MnB5 (36 μm). Finally, the potential benefits of adopting a rapid heating schedule for the coating-free PHS are discussed. [ABSTRACT FROM AUTHOR]

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]

The effect of testing temperatures on wear-induced austenite-martensite transformation and the resultant wear resistance in metastable austenite-containing steels remains unclear. This issue is addressed in the current study, wherein the wear behavior of medium Mn steel was studied using a ball-on-disk sliding test at 20 °C, 0 °C, −50 °C, and −120 °C. The results demonstrate that the strain hardening and toughness enhancement produced by metastable austenite in austenite-rich steel greatly reduce wear volume at low temperatures. The dominant wear mechanism thereby changes from grooving and ploughing to cutting compared with martensitic steel. The wear resistance of austenite-rich steel depends significantly on the thermal and mechanical stabilities of metastable austenite. This study provides the basis for the development of novel wear resistant steels with relatively low initial hardness applicated at low temperatures. • Effect of testing temperatures on austenite-martensite transformation and resultant abrasion behavior are presented. • Austenite transforms into martensite during wear, and the wear-induced transformation is highly dependent on temperatures. • The enhanced strain hardening and toughening lead to the decreased wear volume at low temperatures inaustenite-rich steel. • Thermal and mechanical stabilities of austenite significantly affect the wear resistance of austenite-rich steel. [ABSTRACT FROM AUTHOR]