Abstract
The need for iron-based alloys with exceptionally high strength levels (over 1.8 GPa) is in increasing demand across various industries. This is especially true in ultra- lightweight body-in-white structures for battery-electric vehicles, whereby factors such as crashworthiness place greater development focus on novel hot-stamped steels. However, although such materials are already commercially available their application in conditions where hydrogen might form is limited due to the risk for hydrogen embrittlement (HE). To mitigate this risk, it is therefore necessary to design and develop alloys that contain optimised micro-constituents, which can immutably trap hydrogen. Recent studies have demonstrated that deep trapping sites are considered most beneficial when the hydrogen charging is not continuous but only a one-off internal-hydrogen event such as in spot-welding, hot-stamping, or electroplating. From this perspective, we can appreciate the strong incentive to develop HE resistant press- hardening steels (PHS) which exhibit a large portion of deep hydrogen traps that can irreversibly trap the isolated increase in internal hydrogen. The ideal trap site for decreasing the susceptibility to HE has a large hydrogen desorption activation energy together with a small saddle energy (also termed energy barrier) for hydrogen to enter the trap site. Carbon vacancies at the coherent interface of vanadium carbides (VC) resemble such ideal traps. However, there have been very few studies investigating the integral role of steel processing parameters in optimising critical factors such as the precipitate’s composition, size evolution and number density. Factors such as coil annealing temperature and austenitisation temperature can play a decisive role in the trapping ability of the steel, which is of great importance to steelmakers and heat- treatment facilities.
This work is concerned with optimising of such carbides as a function of process parameters such as coil annealing and hot-stamping heat treatment methods. By contrasting two coiling temperatures, we demonstrate how the carbides differ and how that corresponds to the types of traps formed. The material investigated was model alloy with strength over 1800 MPa containing 0.15 wt. % V. Transmission electron microscope (TEM) was used to evaluate the size and phase fraction of the carbides, while thermal desorption spectroscopy with mass spectrometry (TDMS) was used to deduce the nature of traps formed as a function of coil annealing. In addition, the mechanical performance of the steel was evaluated using constant load test under cathodic charging. All obtained results are compiled to understand the influence of the process parameters to improve hydrogen embrittlement resistance via better hydrogen trapping ability.