The work studies the kinetics of vanadium carbides precipitation in micro-alloyed steels by combining Small angle Neutron Scattering (SANS) with other techniques. They studied the precipitation under different heating conditions, and alongside a phase transition that occurs in the material. Their results can be used as a starting point for modelling precipitation kinetics in vanadium micro-alloyed steels, which may lead to improved steel design and performance for automotive applications.
There are many drivers in the automotive industry for reducing the weight of vehicles, whilst maintaining strength, and the ability to manufacture complicated parts. Steels containing nano-sized precipitates of other elements have the potential for use in these applications. To ensure efficient use of resources, the goal is to lower the amount of additives used, while maintaining the mechanical improvements.
Vanadium is widely used as a micro-alloying element to improve the performance of steel through its effect on the microstructure and, consequently, the mechanical properties through precipitation strengthening. The precipitation of vanadium carbide occurs primarily in one of the phases of steel, ferrite, but not in another, austenite. It is also common in the interface between these two phases. The vanadium carbide precipitation is crucial to the hardening of the steel, and therefore it is important to understand its formation, and its impact on the transition between the two phases of steel.
This study focussed on the rate of formation of the vanadium precipitates in two steels with different vanadium compositions on heating at 900, 750 and 650 °C, and used SANS to give statistical information on the average size, volume fraction, number density and size distribution of precipitates over larger sample volumes.
The group, led by Erik Offerman from TU Delft, in collaboration with TATA Steel, found that, although predicted by calculations, neither precipitation or phase transition occurred on heating at 900 or 750°C. At 650°C, the austenite-to-ferrite phase transformation initiates the vanadium carbide precipitation, and the steel with double the vanadium and carbon concentration showed double the density of precipitates. However, even after 10 hours, neither steel showed the precipitation content expected from calculations.
Their paper – with lead author Chrysoula Ioannidou – provides important information on the kinetics of the vanadium carbides in low-carbon steels and the interaction with the austenite-to-ferrite phase transformation. The results will inform models of the precipitation kinetics in vanadium micro-alloyed steels, and this could lead to improved steel design for automotive applications.
“We are very honoured with the Vanadium award, since IOM3 is an internationally respected institute in the field of materials, minerals and mining," explains Dr Offerman.
The full paper can be found at: https://doi.org/10.1016/j.actamat.2019.09.046
The announcement from the Institute of Materials, Minerals and Mining can be found here.
Previous research from this group featured as an Industrial Highlight in our 2019 Annual Review.