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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]

Herein, the effect of Nb content on the phase transformation kinetics, microstructure, and mechanical properties of hot‐rolled quenching and partitioning (Q&P) steel is investigated. The characteristics of three C–Mn–Si–Ti steels (0.18C, 2.0Si, 2.6Mn, and 0.015Ti) containing 0, 0.027, or 0.061 wt% Nb are compared. Results reveal that grain boundary pinning by precipitates and Nb solute drag effects refine the austenite grain size during the hot‐rolling process; the microstructural refinement is carried over to the final microstructure subjected to the Q&P treatment. The remaining supersaturated Nb suppresses the bainite formation and decreases the final bainite fraction formed in the Q&P process. The microstructural evolution leads to an increase in the ultimate tensile strength (UTS) of the steel containing 0.027 wt% Nb from 1169 to 1228 MPa, while keeping the total elongation at 18%. When the Nb content is increased to 0.061 wt%, the UTS of the steel increases to 1313 MPa, but the elongation at break drops to 16%. The effect is due to the carbon consumption by the Nb precipitates, which causes a decrease in the stability of the retained austenite and reduces the strain hardening at high strain levels. [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]