Researchers unravelled 3D-atomic scale solute partitioning across multiple constituent phases and occurrence of competitive reactions during high-strength steel production

The development of lightweight steel structures is being expedited by the automotive industry to reduce the carbon emissions, and hence, lighter cars must be strengthened in different ways while ensuring their safety and high performance concomitantly. Current strategies for the advanced 3rd generation high strength steels focus towards development of sustainable steels containing multiphase constituents finely divided down to nanometer level through microstructural engineering to accomplish excellent mechanical property combinations and meet the demands of modern engineering applications, where high-strength, lightweight constructions together with enhanced crashworthiness have become crucial.
MME's researchers in front of the university
Professor Jukka Kömi, Professor Mahesh Somani, Dr. Sakari Pallaspuro, Dr. Sumit Ghosh and Dr. S. Assa Aravindh.

The latest development of ultrahigh strength steels has adapted to the new concept of microstructural design involving the quenching and partitioning (Q&P) treatment, which has gained considerable attention due to its ability to impart a superior combination of ultimate tensile strength and elongation to fracture, as stipulated in 3rd generation advanced high-strength steels.

Though amenable for industrial implementation, achieving reliability in controlling the Q&P process on a repeatable basis is in its early stage. It is mainly ascribed to the lack of a comprehensive understanding of the process down to quantum mechanical level. In the case of Q&P steels, the stabilization of austenite is achieved by carbon diffusion from the supersaturated martensite into the austenite, but carbon partitioning is often jeopardized by competing reactions such as carbide formation.

The researchers from Materials and Mechanical Engineering (MME) (Sumit Ghosh, Sakari Pallaspuro, Mahesh Somani, Jukka Kömi) and Nano and Molecular Systems (NANOMO) research units (S. Assa Aravindh, Marko Huttula), University of Oulu in collaboration with the Indian Institute Technology, Ropar (IIT Ropar) (Khushboo Rakha, Shahriar Reza), have employed a modern three-dimensional atom probe tomography (3D-APT) technique to resolve the distribution of substitutional solute elements through the carbide–ferrite/martensite and/or carbide–austenite interfaces. Together with quantum mechanics calculations, the feasibility/formation and transition of stabler carbides and the influence of their atomic configurations for different alloying concentrations have been clearly elucidated. By understanding the interplay between the type of carbide and C distribution at the matrix/precipitate interface, the Q&P conditions could be precisely tailored for a given composition to achieve the optimized combination of mechanical properties.

This work shows great promise in respect of improving the control on the Q&P process through appropriate composition and process designs, thus avoiding the undesirable occurrence of carbide precipitation and austenite decomposition during the processing. This can render expeditious production of advanced 3rd Gen steels using energy-efficient and environment-friendly processes, thus realizing the commitment towards Carbon neutral Finland by 2035, as also desired in the UN Sustainability Goals. Financial support from Jane ja Aatos Erkon säätiö (JAES), Tiina ja Antti Herlinin säätiö (TAHS), and the Academy of Finland (grant No. #311934) for the projects titled as Advanced Steels for Green Planet (AS4G) and Genome of Steel (Profi3) project is courteously acknowledged.

The current research progress strengthens the University of Oulu´s research profile concerning Hydrogen future and sustainable steel. As the members of the Centre for Advanced Steels (CASR), MME and NANOMO research units have made significant contributions to the cross-disciplinary research of the advanced high strength steels, while enabling fruitful collaboration with IIT Ropar.

The work is published in the top science journal Nanoscale.

Article:
Sumit Ghosh, Khushboo Rakha, Assa Aravindh Sasikala Devi, Shahriar Reza, Sakari Pallaspuro, Mahesh Somani, Marko Huttula and Jukka Kömi. A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying, Nanoscale, 2023, Advance Article, https://doi.org/10.1039/D3NR00816A

Last updated: 16.6.2023